A rooftop unit (RTU), also called a packaged unit, is a factory-assembled air conditioner that puts the compressor, condenser, evaporator coil, and supply blower in a single weatherproof cabinet on the roof. Conditioned air is ducted straight down into the space below. It's the default cooling choice for strip retail, offices, restaurants, and small warehouses because it keeps all the noisy, serviceable equipment off the occupied floor and frees up interior and ground space a split system would need.
RTUs sit on an insulated roof curb — a raised, sealed frame that ties the unit to the ductwork and keeps the roof watertight. When an old unit is replaced, a curb adapter lets a differently-sized new unit drop onto the existing curb without re-cutting the roof, which is why like-for-like rooftop swaps are far cheaper and faster than a first-time install.
Capacity is quoted in tons — one ton equals 12,000 BTU per hour of cooling — and commercial single-package units commonly run from about 3 to 30 tons. That's why commercial pricing is anchored on dollars-per-ton rather than the flat equipment prices used for homes: a roofer-accessible 5-ton swap and a craned-in 25-ton replacement are different jobs at very different scales. A contractor who quotes by tonnage, talks about the curb and the crane/roof-access plan, and matches the AHRI-certified model to your load is treating it as commercial work, not an oversized house call.