Phillip Remaker provided an excellent explanation of new IOS release numbering rules in a comment on the Did you notice 15.1T is released? post. Here’s a short summary:
- 15.0(1)M was an exception which consolidated the transition from 12.x rules to 15.x rules.
- Every new 15.x epoch will start with feature releases (15.1(1)T, 15.1(2)T ...) and end with a mature mainline 15.x(y)M release, which will get bug fixes and maintenance rebuilds.
- 15.x+1(1)T will appear approximately at the same time as 15.x(y)M and the whole cycle will repeat.