Profile and Actual Zone

Question

This is a little more of a GD&T standards question as opposed to a PC-DMIS question-- and probably a rudimentary question at that: According to ASME Y14.5.1 2019, to report out a profile, you take the extreme point on whatever side of the true profile it is, and double it to get the "actual zone." This is how profile is to be reported whether we're checking the profile manually with an indicator or using inspection software. My question is why is this the way to calculate it as opposed to adding the highs and lows together (FIM) like a form or orientation inspection? Engineers have asked me about this before and the most elementary way of answering it was to tell them that the profile result was representing what the profile tolerance "would have to be" in order to pass because doubling that value created a new zone (actual MMB and actual LMB). I'm still not entirely sure why. 
 For the record, I never read the actual ASME Y14.5.1 2019 text; I only watched videos about it so there's a good chance that I'm missing info.

Matthew D. Hoedeman · Accepted Answer

it has changed over the years, and has been different between ASME and ISO (it WAS max-min at one point, but if both were on one side, then double the max or min). The way you describe it is 'correct', the reported profile is the minimum required to enclose all data points. By reporting the profile this way, it is the only way to have a SINGLE rule for it. Personally, I think the profile result is useless, at a minimum you also need to report the MIN and MAX. Personally, that also doesn't tell you enough since that one dimension doesn't tell you WHERE any OOT is on the part. I report each point as a point with the correct +/- tolerance based on the profile callout.

NBO CMM · Answer

The profile tolerance on the drawing is given as a total zone that is equally disposed (in most cases) about nominal which means as the tolerance increases on the + side it increases on the - side equally so the zone opens equally until it reaches the furthest point and reports how wide the zone is

Neil Challinor · Answer

It's similar to how position works - for position, you take your deviation from the nominal location (basic dimensions) - which is a RADIAL value - and then multiply it by 2 in order to compare it to the position tolerance - which is a DIAMETRIC value. 
 The definition in ASME Y14.5.1 - 2019 (math standard) is quite complicated, talking about "g" values that grow or shrink simultaneously from each tolerance zone boundary until they make contact with the actual profile. 
 
 If you break this down (compare the ASME definition in black on the left hand side to the blue section on the right of my screenshot) you can see that this is mathematically equivalent to working outwards from the centre of the tolerance zone to find the largest deviation and then multiplying it by two.