Air carrier pilot reported confusion relating to the location of the hold short position regarding taxiway S1 for Runway 27L at PHL.

Narrative

Arriving on Sierra Taxiway from the west and at the entry point for the S1 holdshort; Ground asked the aircraft in front of us if they were ready for departure. They stated no. He directed them to make a left turn on P4 and hold short on M of Sierra Taxiway. We were then #1 at the Sierra/P2/S1 entry point. An aircraft had been waiting on P2 and another on Sierra facing west toward us. An aircraft had taken off before the Ground Controller asked the aircraft in front of us if they were ready for departure. He then directed us to move forward and 'hold short at S1. I asked the FO (First Officer) if he understood that to mean 'Hold Short of the runway on S1'? He stated that he thought he did since he didn't state 'Hold short of S1'. We switched to Tower as directed and moved into position in front of the hold short of 27L at S1. The Tower Controller then indicated to us that we had cut in front of 9 other aircraft. I told him that we were directed to Hold Short at S1 not of S1. He subsequently cleared us to take the runway and takeoff. Nothing further to report.Switching controllers and non-standard radio calls. Unfamiliarity with PHL operations. Use of S1 as the hold point instead of P2 taxiway. On the AMM (Airport Moving Map) chart; S1 and P2 both appear as taxiways. If the Ground Controller wanted me to hold short of P2/S1; I would have expected 'Hold short P2'. S1 on the AMM chart looks like a 'hold short position' and only becomes a taxiway after the hold short position. The standard taxi-way charts seem to indicate S1 as a taxiway versus a hold short position.Make P2 and S1 the same 'taxiway' so there isn't any confusion. Transfer control from Ground to Tower earlier. Utilize more clear clearances. As the Captain; I should have asked the PM to clarify the taxi clearance. Based on the fact that I asked him what he thought the controller's intentions were; should have immediately prompted me to ask for a clarification. I didn't do this; which if I would have; would have easily trapped this error. Lesson learned.