[p2t2p]

It's simple, everything that has existed should behave exactly as it used to before. In order to ensure this people invented lots so technics - code reviews, exploratory testing, integration tests and so on. It's even boring to discuss it, it's just basics.

[t3soro]

He means, how can a machine determine it?

[p2t2p]

Ah, you mean what to bump and when? That's easy, you make a decision to break an API so you need a major version bump. In this case you perform a customized build overriding variables in CI GUI. All other releases are automated minor bumps if on master and patch version bump if it happens on release branch which we keep around for bug fixes. Those bug fixes propagated through release branches up to the master automatically whenever you merge a PR with a bug fix.