The subfloor is the base layer of a floor assembly beneath the finished flooring. In many wood-framed homes it consists of plywood or oriented strand board (OSB) panels supported by joists. It provides the platform for the floor layers above. Underlayment, when required, is an additional layer used to suit the selected floor finish.
A flooring specification should identify both the finish and the base it needs. Ask whether the existing subfloor will be reused, whether repairs are needed, and which additional layers the flooring manufacturer requires. Include those layers when reviewing transitions to neighbouring rooms. The flooring preparation guidance offers related context on getting a surface ready; the subfloor is the base being prepared, rather than the visible floor.