Air carrier Captain reported an inability to verify a taxi clearance due to ATC's use of a taxiway designation not charted on the airport diagram.

Narrative

Upon requesting clearance for takeoff; the Ground Controller informed us of an Air Traffic Control flow time to our destination. This delay was minimal; requiring us to allow two preceding aircraft to depart before our flight. ATC directed us to proceed on taxiway A and; at A1; enter the Customs Ramp. We commenced our taxi from gate. As we approached A1; it became evident that the taxiway was too narrow for safe passage. Consequently; I stopped the aircraft and instructed the First Officer to seek further clarification regarding the taxi instructions from the Ground Controller. The Ground Controller subsequently explained that the Customs Ramp was situated at the end of taxiway A; and the left turn into the ramp was also designated as A1. I made an effort to identify the 'other' taxiway A1; consulting the airport's moving map and the 10-9 chart. However; I was unable to locate this alternate designation.Feeling reasonably confident in our understanding of the instructed holding point; as I had previously complied with it on flights departing from BOI; I proceeded to enter the East Deice Pad and halted short of taxiway A on the Customs Ramp. I did not find it necessary to further contest the matter with the controller; as I believed there was a shared understanding of the intended actions and any discrepancy could potentially be attributed to a charting error. Consequently; we waited for the departure of the two preceding aircraft and executed our departure without incident.