+1 to Olive. Basically P1 indicates your starting point and F1 denotes the first offspring.
From there it gets really muddy. Ideally one separates and moves each generation to their own tank to follow progress and denote each generation there after. However as shrimpers it is not feasible for most of us- so as a result there's a lot of back breeding going on, etc. Because of this we tend to bastardize the process some and make educated guesses as to what generation we're up to now in our tanks.