Flash, actionscript 2.

Does anyone know what the letter 'a' in the pic is supposed to indicate? The one that the red arrow is pointing at.

I ask because the sub-frames under that frame which is labelled 'a' seem to have slightly wonky dimensions when playing it from the .swf file. I was wondering if this mysterious letter a was the cause of it, or if the weird dimensions were due to something else.

On the stage I can tell that the sub-frames look fine due to comparing with the other frames by flicking back and forth, but when ctrl + enter is pressed, certain sections become abnormally stretched out. Thus I deduced that I might have accidentally added a filter or something and wondered if this 'a' marker was an indication of it.

Also, does anyone know if this 'a' thing exists in ActionScript 3? Is Actionscript 3 better than 1 and 2?