A locked rear end has no differential action, so both rear tires are driven at the same speed. A spool is the purpose-built version; welded carriers, automatic lockers and limited-slips are looser relatives often lumped in.
Four different parts get called “locked,” and they don’t behave the same. Wikipedia’s locking-differential article draws the lines: a spool “connects the two axles directly to the ring gear” with “no differentiation side to side”; a welded diff is one where “the gears in an (open) differential are welded together”; an automatic locker such as a Detroit Locker will “lock and unlock automatically with no direct input from the driver”; a limited-slip is “not capable of 100% lockup.” Your rulebook usually picks — IMCA’s 2026 Modified and Stock Car rules both call for a “steel spool (full or mini),” 2026 Hobby Stock says “Mini spools only.” A locked rear is also why stagger matters: with no diff, the bigger right-side tire is what turns the car.
A spool, a locker and a limited-slip drive off the corner differently and are not interchangeable in tech, so "locked" on its own is not enough information.