It is used in industry to measure other gaps... and they would not have any way of knowing if it were one part of a circle it was checking or just one space.... so it measures the space which would be left for oil between our rod and bearing at the point measured. The Mercedes manual gives the dimensions for each item.... and thus you get the gap by subtracting the numbers after careful measuring... but some manuals give the size as measured on the bottom of the bearing cap with the plastigauge.
My old Ford pickup truck has long had a ' thumping' noise in the engine.... when I got to reading about it I found that the factory had sent out some engines with too much clearance on the crank bearings....and specified putting oversized bearings in... and I think this was only on the cap side... to take up the slack. It was also recommended that one purchase the ' higher' grade plastigauage... a higher tolerance than normal... which I would think would be proper for our engines also....