Text objects are introduced in chapter 4 of the user manual and further listed under :help text-objects
, where this is very easy to find:
at "a tag block", select [count] tag blocks, from the
[count]'th unmatched "<aaa>" backwards to the matching
"</aaa>", including the "<aaa>" and "</aaa>".
See |tag-blocks| about the details.
When used in Visual mode it is made characterwise.