Unlike R and theta (the 0 with a slash), which represent radius and diameter respectively, I don’t believe the other letters are industry standard drawing symbols. Rather I suspect they are names for key reference dimensions in the specific drawing you show. Did the drawing come with some supplementary text that explains the terminology? That’s what I would expect.
Another possibility might be that these designations are commonly used in the industry from which the drawing is taken.
I agree with @AlW, they should be internal code from Lego, some of them seems to be related to manufacturing tolerances and others about proportion since the mini figure has to fit with other elements of the Lego system
Those are ‘lego units’. I believe the P stands for plate. A minifigure is 12 plates high. Its head is 3 plates high. A stud is also 3 plates high. A minifigure is thus 4 studs high. See stud for measurements in mm.
Most measurements in the drawing of the minifigure in are mm. I’d just ignore most of the letters here.
Anyway, have a great time reading up on LEGO building blocks over at brickimedia. Also have a look at LDraw for a CAD-like LEGO drawing tool. Also a nice LEGO measurement conversion calculator is at http://studs.sariel.pl/ .