Tower Controller described an aborted take off event noting the coordination procedures contained in local directives are in need of clarification regarding Local Assist responsibilities.

Narrative

A B777; pilot initiated aborted take off on Runway XXL. I was working Local Control during night operations. The airport configuration was landing Runways XX; departing Runways YY. Ground Control had numerous Runway XX departures outbound destined for the Far East and Europe. Ground Control released control of the airport's west end taxiways to my control. By doing that Ground Control was keeping the area past the departure end of Runways XXL and XXR sterile. The B777 was approaching Runway XXL; next arrival was a CRJ2 on a 15 mile final. I instructed the B777 to LUAW on Runway XXL. The B777 said they were not ready for departure; so I instructed the B777 to hold short of Runway XXL and advise when ready for departure. A few seconds later the B777 said they ready for departure. I scanned the entire runway including the west end taxiways; announced to the Tower Cab that 'I'm going west off the left'; did not observe any aircraft; so I cleared the B777 for take off on Runway XXL with the CRJ2 now on a ten mile final. A B737 was arriving on Runway XXR as the B777 was entering Runway XXL and beginning departure roll. The B737 was instructed to hold short of Runway XXL at Taxiway Echo and the aircraft held short of Runway XXL. I observed the B777 aborting his take off run at approximately Taxiway Lima. I departed aircraft off Runways YY R/L as he rolled past the intersection. I instructed the B777 to exit the runway at Taxiway Echo; but he was unable so I instructed him to exit at Taxiway Delta and join Taxiway Bravo; because the CRJ2 was now inside a two mile final. The B777 exited at Taxiway Delta and taxied back to the gate. Pilot stated to Ground Control that he observed a aircraft on Taxiway Z which is beyond the departure end of Runway XXL. The airport SOP Local Assist chapter is written in a way that gives the Local Assist Controller the ability to coordinate the use of airport's west end taxiways with Ground Control on behalf of the Local Controller. These coordinations are accomplished by placing red/white placards on the RACD to the Controller that does not own the west end. The coordinations are currently not performed on a recorded line; because it is understood that the west end taxiways are physically located beyond the departure end of the runways and therefore could never result in a runway incursion. The management team recently had a discussion surrounding this exact circumstance recently. All managers were asked for their input regarding the current west end procedures. This item is still open in the facility. I feel the west end coordinations should be conducted on a recorded line. It protects the personnel involved and it creates a time line indicating what was done and where and by whom. This becomes critical when investigating an event; such as this. The Local Assist Controller should be assisting the operation in following through with coordinations between Ground and Local; but never actually perform coordinations between those two positions. Rewrite the SOP to clearly define Local Assist duties for west end operations.