following-to-join vs concurrent filing

Thank you very much.... however, I am confused about the difference between following-to-join vs concurrent filing.... would you please shed some light?

I assume you said that he is still considered "concurrent filing" because my 485 has not been approved yet, is that right? or, because we were married before I submit my 485?
So, just curious, and also for education purpose, "following-to-join" refers to the situation that spouse's application is submitted after the primary applicant's petition is approved, is this right?