Neither a breeder nor pollen chucker, I'm a humble seed maker, but I dont seem to see a concise answer to the thread...
S1: The first selfed generation.
F1: the first cross between two unrelated parents. The F stands for filial, and refers to the fact that all F1 progeny of the same cross are full brothers and sisters to one another.
F2: the second cross between two of those F1 plants.
View attachment 520584
If you took something to F6, and decided it was finished it would become P1
then using it in a new cross you'd go back to the top of the chart again...
say you took it to F4
then hit one it with one of the original parent plants...
F4Bx1
hit that with its parent again...
F4Bx2
S1 hit with pollen from its parent = S2
Parent hit with S1 pollen = S1BC1 or S1Bx1... I have also seen it written as SB1
even more....