An AvailablePagelet collection is returned by the AvailablePagelet TabDefinition method.
It contains a collection of all the pagelets that could be assigned to the tab. It's an aggregation of the pagelets in the AssignedPagelets collection and all the pagelets in each of the categories in the DynamicCategories collection. Pagelet category sequence numbers, followed by pagelet sequence numbers, then name, sort this collection.
If you have two pagelets assigned with the same name (that is, an assigned pagelet and a dynamic category pagelet), the assigned pagelet takes precedence over the dynamic category one, and is the only one listed.