F1 is the initial cross of two varieties, in theory creating hybrid vigor and a wide genetic profile. These seeds would be grown out to find a particular phenotype, You can also use them in your farm. Growing 50 f1 crosses outdoor can provide an ample breeding stock for the next summer.
F2 and F3 are the same varieties back crossed, imo, its best to get seeds that have been open pollinated as they will have more genetic variety. If you are running an indoor crop though you'll want to choose an F2 or F3 variety from a reputable seed provider, do your research.
Now onto S1's
S=selfed; i.e. chemically treated to mutate, to cause sexual organs of the opposite sex to appear and results from those are used. Typically it's a female that is 'selfed', that is to say chemically treated so as to cause viable male organs and pollen to be generated. By that pollen the original female is bred. The numbers denote generations.
I am likely not using the proper terminology, the above explanation is my take on what I've read on the subject.
S1=Femenized Seed
They dont really exist, S1's and Feminized seeds only exist in the pot grower world. I was in a horticulture class and a guy asked the professor about feminized seeds, the professor laughed knowing what he was asking about and said, Well I dont know what process did they use to make them all females?
The guy replied with "I don't know"
Me being me, I said, "oh don't they use a hermaphrodites pollen with a female to produce the seeds? It creates an XXXX."
The professor then went rambling about how that's one way to do it, and how it will majorly narrow the gene pool.
So if you walked into a room full of botanists, and asked "Whats an S1" They would laugh at you. An S1 is the closest thing to a genetic copy you can get. Cali connection seeds are all S1's to my knowledge, Swerve takes the clone, hermies a branch and pollinates the room with the branch.
Grow a plant, Take a clone, Hermie the clone so it produces pollen with female haploid cells and pollinate the female with that pollen.
I hope this help explains things?
Chobble
Edit:
Also I did a quick search to make sure I wasnt being an asshole, the farm is one of the main references to S1 with icmag etc. I've seen it around all the pot grower sites but never in a text book etc. Upon further reading there is also evidence that supports what I'm saying.
Heres a post about jaws where they talk about "Creating S1's"