DigiTech PS200R-240-B..but 3VAC too!?

I got the above mentioned PSU for a RP55 efx pedal, the main output is 9.6VDC 300mA so that's all nice and sorted, however... the mystery is, on the side of the block (wall wart) is also a tiny power socket (about 4mm dia.) with 3V AC @ 100mA - anyone know what that was intended for?

The HarmanPro Group MFG P/N: SB41-096-0300D

I'm that sort that needs to know everything.