I loaded a 3rd party song file and the chord representation in the file was showing some chord notation that I haven't seen before.
Example:
Am.pgs and Am...d

Can someone explain or point me to the related documentation? Thanks!

Sure. They are playing instructions for the chord. Look in Help, Index, Rests

Am.pgs means to play the Am chord but REST (one period) all instruments EXCEPT (p)iano, (g)uitar and (s)trings

Am...d means HOLD (three periods) Am EXCEPT (d)rums

The other one not in your example is two periods = a SHOT (a quick hit)

And one more thing to learn that you will see: there are PUSHES with one or two ^^ characters before the chord name. A PUSH plays the chord early.

Great. I knew about pushes, but these were new to me. Thank you very kindly, Matt!