i'm pretty sure its the rgb 16 bits/channel.
and just to add, if you need extra guidlines you can add
767 (center of back)
1620 (dead center)
2473 (center of front)
EDIT I have since realised that you can drag a guideline out and right click on the guideline where it meets the ruler to bring up a properties box, then simply tpe in where you want the guideline
