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:
https://www.ciscozine.com/an-introduction-to-cisco-ios-version-15/
Some good coverage of licensing:
https://www.cisco.com/c/en/us/td/docs/routers/access/sw_activation/SA_on_ISR.html
https://learningnetwork.cisco.com/docs/DOC-20321
https://www.cisco.com/c/en/us/td/docs/ios/csa/configuration/guide/csa_commands.html
New for IOS-XE 16.x