- Root site collection: container for portal, home sites
- Explicit site collection: container for department, services, regional sites
- Wildcard site collection: container for team, community, project sites
- Top-level site (site home): container for content and subsites and channels
- Subsite (site section): container for content and even more subsites and channels
Structures of top-level sites and subsites are generally labeled as wide or deep. A wide site structure uses multiple TLS sites (and hence multiple site-collections), while a deep site structure rather uses a single TLS site with multiple subsites with even more subsites. See the SharePoint site classification & structuring post for more details on this and the above five containers.
However, not all combinations are viable due to SharePoint software boundaries for the containers. This especially applies to how to combine managed site-collections with wide and deep site structures. As outlined in my last post, each major category gives a recommendation on the usage of wide and deep site structures based on the site classification. The recommendation on how to combine wide/deep with explicit/wildcard site collections is shown here (click to enlarge):
As you can see, it is not recommended to combine explicit site-collections with a wide site structure due to the limited set of explicit site-collections per SharePoint web-application. Using wildcard site-collections give the most flexibility as there is almost no limit on the number of site structures wildcard can hold. If you need a new top-level site, just create a new site-collection.
The SharePoint software boundaries on site structures per web-application is shown here (click to enlarge):
Each SharePoint web-application can hold about 20 explicit site-collections and about 50000 wildcard site-collections. About, as these boundaries are not hard limits - but performance will suffer significantly should your site structure push beyond them. The recommended limits on subsite structures is about 125 second level subsites under a top-level site, each with about 2000 third level subsites, for a theoretical maximum of 250000 subsites per site collection. Theoretical, as the maximum recommended database size per site-collection is 100GB and with hundred thousand subsites there will be little room for content in each subsite.
Note that root or explicit site-collection storage cannot be scaled as you cannot add another content database to a site collection. If scalable storage is a requirement, using wildcard site-collections is best practice.
Respect the SharePoint boundaries, and I recommend your never get close to pushing them. I once broke these recommendations myself for a wide site structure; a couple of years later the "client site" solution started behaving erroneously (mainly timeouts) because of more than 60000 clients being provisioned by then, pushing the 50000 TLS limit. The next time I designed a site structure to hold a million client sites, we did a proper information architecture LATCH (Location, Alphabet, Time, Category, Hierarchy) analysis. This resulted in a deep+wide site structure with the one million subsites (deep) hierarchically distributed based on the 9-digit customer number, across multiple site-collections (wide).
Steve Goodyear's decision diagram in the Determining Between SharePoint Site Collections and Sub-Sites post shows other important technical aspects when deciding between wide and deep.