depends entirely what frame you have!
I have a steel hardtail with round section tubing onto a square plate dropout (like jsut about anything else in teh budget bracket), bought one from KMB at full price as I just needed it (rim didn’t have much life left in it on the back so needed to swap onto a spare disc rim). they said they’d help work out how to fit it, which naturally never happened, and when I returned they wanted a stack of cash, when in the first instance they said ‘buy at full price, we’ll help you fit it for the price’. Going back on their word & forgetting conversations seems to be an all too familiar trate with them (on warranty items etc).
if your frame has a fairly square shape on the seatstay and your dropout is again fairly square then chances are it will fit and stay put resonably well. just don’t go expecting to be able to fit hope 6pot’s to your bike!! get the lowest powered caliper & smallest rotor that will fit, otherwise it will just twist the thing off the bike.