Most case ceiling but this part for smoke alarm placement this you will have to read the manufacter installment sheet for this and by the way the specfic rules about smoke alarm generally will be found in IRC ( International Resdential Code ) plus there will be your state / local addtional code requirment it may show up.
But try not to put it near the bathroom or kitchens that the two are they will trip by cooking smoke or steam from the shower / bathtub will trip them.
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