Here are the few that I use:

X = cutoff or waste side of a line.

O = Show face, more importantly used as the reference face. For instance ,
if I am using a tennon jig, I always reference the front of the pannel. If
there happens to be any thickness irregularities, the will show up on the
back of the panel.

Triangle -drawn across a joint to show the order and orientation of the
components of a glue up pannel.

Right angle (like in geometry class, two lines to form a box in an interior
angle) - used to indicate the adjacent sides of a board which were jointed.
When rejointing previously milled stock, it's not always obvious what faces
were trued.

Cheers,

Steve