HO/OO is a designation that really means HO. In England, it is used for OO scale models, but isn't quite right.
I can't find SL100X inthe Peco catalog. SL100 is the basic Peco track; SL100F is the fine-scal code 75; SL8300 is the code 83 "American" style.
SL102 is concrete sleepers (ties). SL100B is concrete sleeper, brass rail; SL100GS is galvanised steel. Aboid the last 2 if you see them.