I only know about the Airbus A300 but I think the later Airbuses are basically the same.
SRS (speed reference system) is a Flight Director mode (not Auto Pilot) used during take off, that gives pitch commands through the F/D pitch bar to maintain V2 +10 with all engines operating and V2 with one engine inoperative.
The V2 speed is derived from performance calculations and is set on the ASI with the speed selector. When the "Go" (TOGA) levers are triggered on the power levers at the start of the take off the Modes "N1" and "SRS" are engaged and this is followed until acceleration (3000ft. AGL) or A/P is engaged.
Hope this helps.