At my current airline, when you apply and accept a ground or sim instructor position, your seniority and longevity accrues. So if and when you go back to the line, your seniority is intact.

In other words, if I were hired/ merged or whatever way into American, I might want to go into the training dept since I live in Texas, and then, when I'm senior enough to hold DFW, go back to the line.