Nailed it!If you're setting it for a pen, set it to A so nothing on the cannelure shows.
Set it to the OAL of the finished cartridge if you're doing reloading, and the type of crimp (anything from no crimp to a roll or factory crimp) so that it crimps into the cannelure. A cannelure is somewhat limiting on the type of bullet vs the overall length of the cartridge, a non cannelured bullet gives greater flexibility for seating depth and casing length.
Monty,
You can get bullets with no cannelure on them so the depth is less critical and you can set the length to what you need. Most of the .30's I load have no cannelure.