Depending on your development, a number is assigned to
genital development (G) and a number is assigned to pubic hair
development (P).
For example, if you have just started growing hair you might be stage P2. It's possible to be P2 and still be G1. Penis growth doesn't start occurring until G3. Genital development begins when your testicles start growing (G2). Most boys don't notice this and so they think they haven't started puberty yet.