Is this an opening decision, or am I facing one?
"Opening" means nobody's acted — is your hand wide enough for this seat? "Facing a raise" means someone before you has already opened — do you have enough equity against their range to continue?
Uniform deals every hand with its real, natural frequency — closer to an actual session, but most hands you're dealt (AA, 72o) were never really in question. "Near the borderline" deals more hands close to the actual raise/fold or call/fold line, so more of your reps are genuine decisions.
Scales how wide everyone's ranges are, opening and continuing alike.
To keep setup short: raises are always sized to 2.5 BB, blinds are 0.5/1 BB, and the facing-a-raise branch only tests call-vs-fold — 3-betting has its own fold-equity math that isn't modeled here yet. Opening decisions are raise-vs-fold (or raise-vs-check on your option when it's limped to the big blind).