The ACR Standard (Assessment, Cleaning & Restoration of HVAC Systems) is NADCA's written procedure for how a duct system should be inspected and cleaned. It defines source removal as the required method and sets what 'clean' actually means.
Under ACR, a proper cleaning puts the system under continuous negative pressure and mechanically agitates every accessible surface so debris is pulled out — not blown around. It covers the whole system: supply ducts, return ducts, the main trunk lines, registers, and the air handler. A cleaner who only does the vents you can see is not cleaning to the standard.
You won't read the standard yourself, but you can ask whether a provider cleans to it. A shop that references the ACR Standard and documents a per-component scope is telling you it follows a repeatable method rather than improvising a quick pass.