Air carrier Captain reported a lack of designation of boundary line; and that taxiway lines were barely visible during taxi at ORD.

Report 1 | Narrative: Taxing on SP; and we were told to hold short of the boundary line. Listening to traffic ahead; they are switching over near FW/A7. Got clearance to go onto SE and then told to continue SP and contact Ground. Contact ground and cleared to A13 with hold short V(this is the 3rd change to our clearance in about 3 minutes). At this point; Ground tells us to hold short A7 (we're turning onto Alpha). Talked to Tower and Tower advised that we had missed an instruction to hold short of the boundary line. Advised tower that we thought the boundary line was the communication boundary line abeam FW/A7(Chart 20-9B).

Tower advised me that no the Boundary line referenced is for the Non-Movement Area on SP with implied instructions to hold short of Alpha(Ground Markings Not on the Chart). Asked for clarification on where the boundary line was they advised SP and Alpha intersection. Contributing factors was construction and taxiway lines barely visible at the intersection with construction on the north side of alpha taxiway. Unfamiliar with where the non-movement boundary line starts; and Taxiways begin; because we were on a taxiway (SP); we incorrectly assumed the communication boundary line on Chart 20-9B was the non-movement boundary line and not a taxiway marking.

Suggestions: In chart 20-B; reference the boundary of where the non-movement area starts; it is not clear on any of the taxiway charts where the non-movement area ends. Ramp control should also use standard phraseology. 'Taxi SP hold short of Alpha' instead of telling us to hold short of a boundary line that is marked on the taxiway and is faded from construction and aircraft movement.

Report 1 | Synopsis: Air carrier Captain reported a lack of designation of boundary line; and that taxiway lines were barely visible during taxi at ORD.