Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Situational Awareness; Confusion
Conflict Ground Conflict; Less Severe
Flight Crew Became Reoriented
Chart Or Publication; Airport
Upon landing our gate was occupied. Told to hold in PAD A. Captain and I reviewed the pages for the hold pads; but it only displayed the Scenic Hold Pad. It did discuss other hold pads in general by saying 'entire aircraft will be contained inside the boundary line'. In pad A we were confused what line this was. There was a dashed line closer to the taxi way and one much further away that looked impossible to stop Aircraft X behind as there didn't appear to be enough room. We chose the former and it didn't provide enough room when Aircraft Y taxied by. His wingtip came uncomfortably close to our aircraft.The pages are not clear as to what the boundary line actually is.  Easy fixes are to put a note with an arrow on the Taxi chart saying to make sure to hold behind the boundary line. If that's too hard to do; make it a hotspot. At a minimum; pages need to show all hold pads and clearly show what the boundary line is.
Air carrier First Officer reported there are two possible boundary lines for the pad area at ORD airport and the chart does not specify which one to use.