Those would be F2,
You would be breeding F1 with F2… Making BX1 (back cross). Useful for stabilizing traits.
Breeding two different strains creates an F1.
Breeding two of those F1 seeds (M & F) will make F2. Breeding two F2’s make F3, and so on. Introduce a different strain, and its back to F1.
S1 is usually referring to Selfed plants, i.e. eliteXelite, rhodelization, CS or STS reversals. These are feminized as there are no males involved after reversal. Its female genetics pollenating female plants.