Here's a fun brain teaser regarding Profiles AND Datums!

Question

ok. I have a simple bracket type part with some flat parallel and perp surfaces. There is a PROFILE callout to -A,B-. When reporting using both -A,B-, profile is OOT. When reporting using just -B-, results are conforming. 
 ALL surfaces are perp and parallel to each other within .001. 
 Why does -A- have such a massive impact on profile assuming -A- is 1) flat within .0005''(inches) 2) perp to -D- within .0005'' (inches) 
 
 Results shown are reported in metric. 
 when using -A,B- 
 when using -B- only

Neil Challinor · Accepted Answer

When you do the profile to A, you are really only capturing the perpendicularity because A can only control two rotations (think the LEVEL) and translation along it's axis (which will have no effect on this particular profile anyway). . 
 When you do the profile to B, you are capturing parallelism and the distance between B & your surface. 
 . 
 When you do the profile to A|B (as it's supposed to be done), you are introducing additional constraints to the DRF because it first rotates and translates to A, then rotates and translates to B whilst maintaining it's orientational relationship to A - imagine sitting the base of an angle block on the A surface and then sliding and rotating it until the side face is making maximum permissible contact with B. From this DRF you are then capturing the perpendicularity, parallelism, form and location of your profile surface.

bfire85 · Answer

Profile to B is already near max. By combining the two, you are now wrapping in perpendicularity, parallelism, and form and location. Seeing you are already near max with just to B. It makes sense.

(And I see Neil chimed in with pretty much the same thing with more eloquent language.)