(TT&TO) : A clearance was a document that permitted a train to pass (clearance by) a train order signal displaying stop and receive train orders. It also was a “packing list” that listed the numbers of the train orders to be delivered at that station. That way a crew could ascertain that they had received all the train orders they were supposed to get. In addition, some railroads added sections onto a clearance that were a register check (equivalent of a form V train order) or allowed clearance by a restrictive signal in manual block territory.
(Train operations): Checking of the physical size of a train across its route. Wide, tall or heavy loads will have to be cleared by a railroad across a route, possibly restricting the speed, which track, whether there can be equipment on an adjacent track at specific locations or mileposts.