Underlayment is the layer installed before the surface you actually see: on a roof it goes down over the deck before the shingles, on a floor it goes down over the subfloor before plank or laminate flooring. In both trades it is bought as rolls of nominal coverage, which makes it a takeoff line item with its own arithmetic. The roofing default is a synthetic roll like GAF FeltBuster — 48 in × 250 ft = 1,000 sq ft, a 10-square roll — while flooring underlayment is counted as representative 100-ft² rolls against the net floor area.
The estimating subtlety is that nominal coverage is not laid coverage. A synthetic roofing roll consumes part of itself in the required side and end laps, so the net area it actually covers is a little less than the printed 1,000 sq ft — sizing the rolls from the same waste-padded area as the shingles is what absorbs the difference. Product matters as much as arithmetic: old-fashioned #15 felt yields only about 400 usable square feet per roll, so a takeoff seeded for synthetic coverage must have its roll value edited down before it prices a felt job.