A cyan line with an "A" in the Level Sequencer

I noticed this cyan line with an “A” on the the sequencer timeline. What is it and what is it for?

First time posting and I tried searching the forum, could not find anything about this. Can’t find anything in the documentation, maybe because I have no clue to what is it called in the fist place
Thanks in advance

It’s a Frame Marker. Purely for lining things up and identifying frames:

Ah, that make sense and would helped a lot more if I knew this earlier. Actually it would make a lot more sense if it is a “M” instead of an “A”
Anyway, thanks for the quick reply. Really Appreciate it