Certificates on the market can be broadly categorized into three types: single-domain certificates, multi-domain certificates, and wildcard certificates. While all can achieve HTTPS encryption, they differ significantly in flexibility, price, deployment costs, security strategies, and renewal/maintenance. Inappropriate selection can not only increase enterprise costs but also lead to complex installation, chaotic management, or limited future expansion. Therefore, a clear understanding of the business model and future plans is essential before making a choice.

　　Single-domain certificates are the most basic SSL type, protecting only a single fully qualified domain, such as www.example.com or api.example.com, but not other subdomains or the root domain. For example, a certificate for www.example.com cannot be used for example.com or blog.example.com. This type of certificate is best suited for pure single-site applications, standalone API deployments, or projects with clearly defined business isolation. Because of its singular purpose, it is the lowest priced of the three types, and has no special requirements regarding compatibility, installation methods, or trustworthiness. It is the most common and hassle-free choice for personal blogs, small official websites, e-commerce showcases, or outsourced independent projects. However, single-domain certificates also have significant drawbacks—as business expands, frequent certificate additions or re-applications are required, leading to continuously escalating long-term and management costs. Therefore, they are only suitable for scenarios with a very small number of domains that don't change much.

　　Multi-domain certificates, essentially SAN or UCC certificates, allow multiple different domains to be bound to a single certificate. For example, they can simultaneously protect www.example.com, blog.example.net, app.example.org, etc., and even different main domains can coexist. These certificates are ideal for companies with multiple independent projects that don't want to purchase separate certificates. Compared to single-domain certificates, multi-domain certificates significantly reduce management complexity and allow for more centralized server configuration. However, their limitation lies in the fixed number of domains in the certificate. It might be purchased with 3, 5, or 10 domains; if additional domains are needed later, a new certificate must be issued, potentially causing business interruption. Furthermore, if the certificate is installed on multiple servers, the update cost amplifies simultaneously; for example, a single renewal can affect all bound services. Therefore, while management is simplified, the risks are integrated.

　　Wildcard certificates are designed to cover an unlimited number of subdomains. A single *.example.com certificate can protect all first-level subdomains such as www.example.com, cdn.example.com, and img.example.com. However, it does not cover multi-level subdomains; for example, a.b.example.com cannot be covered by this certificate unless additional configuration is performed. Wildcard certificates are suitable for internet companies with multi-service, multi-module, and multi-server architectures and rapid subdomain expansion. For example, when a SaaS platform assigns an independent second-level domain to each customer, a single certificate can prevent the need for additional SSL deployment each time a new site is created. Compared to the previous two types of certificates, the biggest advantage of wildcard certificates is their deployment flexibility and scalability, but this also makes them more expensive, typically costing 5 to 10 times more than a regular single-domain certificate. Furthermore, while some free certificate authorities support wildcards, their issuance restrictions and ACME DNS verification are complex, making them unsuitable for projects with zero maintenance requirements. In addition, wildcard certificates also pose security management risks—once the private key is leaked, all subdomains will be affected. This is similar to the risks of integrating multiple domain certificates. Therefore, it is recommended to use HSM or dedicated hardware storage for keys.

　　When choosing a certificate, in addition to the business structure, the certificate verification type must also be considered. Whether it's a single domain, multiple domains, or wildcard domain, you can choose from three verification levels: DV, OV, or EV. DV (Domain Verification) is the fastest to issue and has a high degree of automation, suitable for small and medium-sized websites and automated operation and maintenance deployments. OV (Enterprise Verification) includes company identity information, making it more reliable in B2B systems. EV (Enhanced Verification) was once used for the browser's green bar display, but the special UI identifier has been removed; however, it is still used in finance, payment, and other fields. The price difference of certificates comes not only from the number of domains but also from the verification level. Therefore, if it is just a regular website, there is no need to insist on purchasing OV or EV, as the cost is often more than 20 times that of DV.

　　Some companies easily misunderstand a key point when comparing certificate types: "Wildcard certificates seem to be the most versatile, so can buying one solve all problems?" In fact, if your domain name system spans multiple primary domains, such as example.com, example.net, and example.cn, even wildcard certificates cannot cover multiple primary domains. You must purchase them separately or use a multi-domain certificate approach to add all domains to the SAN list. Therefore, wildcard certificates are suitable for single-primary-domain, multi-subdomain scenarios, not for cross-suffix domain businesses. If your business model involves "platform + multi-tenancy + dynamic subdomain binding," wildcard certificates are indeed the best solution. However, if you have a small number of projects and the domains have different suffixes, a single SAN multi-domain certificate is more economical.

　　Besides type differences, renewal methods and deployment costs also affect certificate selection. For example, single-domain certificates are typically deployed on a single server, while multi-domain and wildcard certificates may need to be deployed simultaneously on multiple nodes, a CDN, an API reverse proxy cluster, and other sub-service systems. If a certificate is about to expire, all environments must update their private keys and certificate chains; otherwise, access errors will occur, creating centralized maintenance risks. Therefore, in multi-domain certificate scenarios, if a certificate expires unexpectedly, the impact may be far greater than with a single certificate system. To mitigate these risks, an increasing number of enterprises are using automated update tools such as Certbot, acme.sh, or CDN-based automated certificate hosting solutions to reduce manual maintenance.

　　Cost is also a key factor in many enterprises' decisions. Single-domain certificates may cost only tens to hundreds of yuan per year, while multi-domain certificates typically increase in price with the number of domains. Wildcard certificates, on the other hand, often start at nearly a thousand yuan or even higher. For branded certificates, wildcard EV certificates can cost tens of thousands of yuan, while free certificates lack OV and EV options. Therefore, if only one or two subdomains need to be deployed, there is absolutely no need to choose a wildcard certificate; otherwise, the cost is significantly wasted. Some enterprises choose to deploy internal system certificates using a "self-built CA" approach, but these can only be used within the enterprise's LAN or self-controlled terminals and cannot be used for public network business; otherwise, they will inevitably be blocked by browsers.

　　If certificates are viewed as a resource management method, then the selection should not be simply based on price, but rather on long-term operating costs. If a website may expand to 30-50 subdomains in the future, purchasing multiple single-domain certificates initially may seem cheaper. However, the maintenance burden in areas such as renewal, debugging, certificate chain updates, and disaster recovery will be enormous, ultimately resulting in a higher actual cost than a single wildcard certificate. Conversely, if there is only one fixed business domain, even if new business is added in the future, it's advisable to allocate new sites to different main domains rather than forcing them to belong to the same certificate system.

　　In summary, single-domain certificates are best suited for independent businesses or simple websites. Multiple domain certificates save on management costs but concentrate risks, while wildcard certificates offer extremely high scalability but are expensive and carry a greater risk of key leakage. The truly suitable solution must be determined comprehensively based on future business plans, budget, security strategies, deployment architecture, and certificate lifecycle management systems, rather than making decisions based on the simplistic logic that "the more expensive the certificate, the better."