It's an alternate layer for another cel in the sequence.
It seems to be used when there should be a cel that should
match one or several cels in a similar sequence.
For example if the exact same scene should be done once
with one kind of lightning and a second time with a
differant.

Or if the exact same scene should be done once with
a character and then with another character.

In those cases, each cels with match a similar but
differant one in the other sequence.

Kinda hard to explaine but look at :



This whole scene is done two times. Once with Sylia
and once with the other girl. For each cel of the
sequence (A1, A2, A3...), there is an exact matching
cel with the other character (A1', A2', A3')...

Cheers,
Yann Stettler