The aisle that exists, not the aisle on the plan
A drawing shows the distance between racking uprights. The floor shows something else: pallets overhanging their beam edge at the front, cartons broken down at the pick face, empty stillages parked at the end of a run, and a strapping bin nobody agreed to place there. Each of those reduces the width available to a machine, and a machine that cannot pass simply does not clean that run at all.
Recording the real figure protects both parties. If the aisle narrows because a new product line arrived on deeper pallets, that is worth knowing before a service is booked against a method that no longer fits. A scope that names the assumption behind each run makes the change visible instead of leaving a crew to improvise in front of a stocked bay. A written assumption is also something a new site contact can read, which matters when the person who agreed the scope has moved on.
