Using the dial on the right slideout of the TDC, the dial is numbered 0 to 18, with 'P' on one side and 'S' on the other. This corresponds to 0 to 180 degrees, Port or Starboard. It is marked 0 to 18 because the dials are too small to write it out.

If you're asking what the Aob means in the computations, the Aob is input instead of the targets course. One can't see the targets course by just looking at it, but if you know the Aob, along with your course and the bearing, you can compute the target's course. This is why it is needed. If you were firing on a unmoving target, you could dispense with the Aob.

I hope this answers your question.

I wouldn't get too hung up on the Sub School training mission. Having to torpedo a suddenly appearing cruiser is a very unrealistic type of training. Contacts were usually attacked after several observations and careful plotting, not like in the School mission.