The Desktop tag library can be viewed as a wrapper of PAPI, ProviderContext, ContainerProviderContext, and specific container building-block providers in the Portal Server software. Thus, a hierarchy is implied.
For example, the ContainerProviderContext (interface) extends ProviderContext (interface). When you use a tag in desktopContainerProviderContext.tld, it also make sense to use it in desktopProviderContext.tld. Similarly, when you use a tag in desktopProviderContext.tld, it also makes sense to use provider tags in desktop.tld because ProviderAdapter implements Provider. By putting the tag in the level that provides the most use, you will not have to make duplicate tags.
At the bottom of this chain are the specific containers (single, table, and tab). Because all these containers extend JSPContainerProviderAdapter, they can use tags in their respective TLDs, as well as tags in desktopContainerProviderContext.tld, desktopProviderContext.tld, and desktop.tld.