Linux kernel are numbered as followed
Major = the major release level
Minor = even numbers are production ready – odd numbers are development versions
Level = sub level of the minor level showing which level of fixes have been applied
So, version 4.2.7 (say) can be installed into production but 4.3.4 (as an example) cannot, even though its a higher number than 4.2.7, as the minor verison is an odd number. 4.3.4 should only be used for testing and development.
Easy when you know how !