Cisco licensing is as best a mild irritation, but in reality, an impossibly poor, yet growing set of ill-conceived disparate implementations. IOS-XE and IOS v15.x licensing is no exception.
Some useful links to wade through the confusion on the various (and ongoing) changes are listed below.
A useful overview of 15.x (now somewhat dated but still good:
Some good coverage of licensing:
New for IOS-XE 16.x