When to change a PCB number and when to change the Revision?
I believe it's just with every other version, change the revision when there is a bug fix, change the decimal when you've made a minor change, change the first number when you've made major change. But there might be other guidelines as well! 

This answer describes the same way as with Software versioning, with the only change that the revision number is changed into a letter. In software versioning, the version consists of a major, minor and revision number: 1.0.0 for example.