You're definitely right. At mainline it's a seperate seniority list, so you're essentially an FA or SD, depending on your personal preference and seniority.

At Jazz all FAs are qualified as SDs, so it's decided on a trip by trip basis, with the senior crew member getting to pick their position. Generally the senior FA will take the SD position due to higher pay, however not always. I do enjoy stepping back and taking the FA position to work Y class instead of J from time to time.