33RS
3/8 pitch, .050 gage , Rapid, Full Chisel.
Good chisel chain
No other numbers, not a safety or low kick back chain. ;
comes with a yellow link & a safety warning.
RS3 with the humped drive links to reduce kickback is what they are pushing for non professionals.
Green mark, Low kickback
Actually it get's much deeper than this.
Available in .325, 3/8", and .404 (not every type is available in all 3 pitches/gauges)
RS/RSC = Chisel chain (aka. Full-chisel), yellow label
RS3/RSC3 = Chisel chain, low-kickback, green label
RM/RMC = Semi-chisel (similar to but not the same as "chipper chain" chain although the term is often used interchangeably),
yellow label
RM3/RMC3 = Semi-chisel chain, low-kickback, green label
RD = Carbide chain
RDR = Rescue chain, which is basically RD chain with the RM2 (
) tie straps.
And that's just the bigger stuff. Then you have the Picco series, with similar nomenclature.
PM/PMC was yellow label Picco (3/8" Low-Profile) chain that was freakin' excellent stuff for small saws. I hear that it is no longer in production.
PMMC3/PMM3 = .043 guage narrow kerf semi-chisel low-kickback chain usually found on Stihl saws under 35cc.
PM3/PMC3 = is the current semi-chisel, green label, low kickback chain that comes on most Stihl saws under 40cc .
PS3 = relatively new,
chisel chain, albiet green label, low kickback. Really wish I had a saw setup for Picco chain, want to try this stuff. C'mon Stihl, give us a yellow label version of this stuff,
PLEASE???? 
and I'm not going to touch the semi-skip, full-skip, square ground, or Klassic stuff.
"Yeah uh, lemme get a 33RSLHK-105 and two 33RS-72's please."
"Would you like fries with that sir?"
) and the Bar And Chain catalog available via their website's Interactive Catalog section. I'd love to see one that included discontinued lines as well.
