An altitude deviation occurred when a widebody flight crew was descending via the KERUP 3A STAR to UAAA.

Narrative

Approaching UAAA on the KERUP 3A. After passing TIRBA at FL148; we requested and were given direct ALOLI. ATC also instructed us to descend to 900 meters. We were switched to a different Approach Controller and we queried him if we were cleared for the approach; as ALOLI was our clearance limit. He responded with 'maintain 900 meters' and there was confusing communication about where we were to proceed after ALOLI; and the call came again to maintain 900 meters. In this confusion; I dipped below 900 meters and corrected immediately. Vector was then given to final and uneventful landing was accomplished; Runway 5L. Taxi to chocks was uneventful w/no comments of the event from ground control or tower. What caused the problem: Taking the direct ALOLI. We should have stayed on the arrival; putting the onus on the controller for our instructions out of ALOLI. Hand flying when confusion started. Captain continuously switching FCP from feet to meters and back without telling me. Allowing myself to get distracted.