What constitutes "significant changes" when releasing a new TestFlight build that has passed beta review? Is there any documentation or first hand experiences as to what constitutes significant changes to an app when submitting a new TestFlight build that has already passed beta review? 

Apple asks: "Have you made any significant changes to this build since your previous submission?"

We added some in app purchases which turn on/off some features but those features already existed in previous submission so maybe this change isn't "significant".
Significant changes would be adding a new code SDK, new advertising, new IAP purchases - basically anything that isn't just a little change to some refactoring or some new cosmetic changes.

If you are changing tens of lines of code and tens of classes - that seems minor unless your code base is extremely small.