Suppose you bought PHPKB version 8.0. That was the major release. You've had it for 14 months now, and you let the support and maintenance period run out. Then we release 8.0.1. Even though your maintenance package has run out, you can still upload and install release 8.0.1. Once your maintenance package runs out, you can only get minor release upgrades for your current version. If you are using release 8.0 and are out of the support period, then you can only upgrade to releases 8.0.x. You cannot upgrade to, say, release 8.5.x (unless you extended your support and maintenance package first). Major versions implement new functionality. Minor versions are for fixes and small tweaks to the system.