Illegal Participation

[john_faz]

My crew had a play this past weekend where the defense had 12 players on the field for a down. We caught it after the ball went live then the running play went for five yards. I was unsure where to enforce the penalty from. We ruled a previous spot enforcement and replayed the down. I thought this was correct but then I check the rule book and noticed that some illegal participation fouls are from the basic spot and others from the previous spot.

Does anyone have a good explanation as to why some Illegal Participation fouls are enforced from the basic spot and others are from the previous spot?