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/products/collateral/ios-nx-os-software/ios-software-release-15-0-1-m/qa_c67_561940.html

https://www.cisco.com/c/en/us/td/docs/routers/access/sw_activation/SA_on_ISR.html

https://www.cisco.com/c/en/us/products/collateral/cloud-systems-management/software-activation-on-integrated-services-routers-isr/white_paper_c11_556985.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

https://www.cisco.com/c/en/us/support/ios-nx-os-software/ios-xe-16/products-licensing-information-listing.html