The model numbers do not follow a specific set of guidelines. However, here are some general explanations:
13= NCE model number designation
SR= Silent running
N= N-scale size but not strictly for N-scale
P= 8 pin (4x2) NMRA plug
J= 9 pin inline JST harness plug
D13SRJ = Silent running decoder with 9 pin plug
One thing to keep in mind is that the front and rear lights are each considered separate function outputs.
So, a four function decoder has a front and rear headlight and two other outputs that can be used for anything else.