Just in case you couldn't follow the discussion in the thread mentioned by Mike, here's the short version:

The detailer can use a generic welding symbol with only the prequalified joint detail listed in the tail, i.e., "B-U5" for instance. That being the case the nominal values listed in the AWS code are used by the fitter to assemble the joint, i.e., 45 degree groove angle, 1/4 inch root opening with backing etc. We all know that the fitter isn't going to be dead on, so the inspector uses the "as fit-up" tolerances added or subtracted to the nominal values listed. The as fit-up tolerances would allow 40 degrees to 55 degrees for the groove angle, 3/16 to 3/8 inch for the root opening and still be acceptable.

If the detailer sees a situation where a larger root opening may be beneficial or a larger groove angle, etc. he can take advantage of the "as detailed" allowances. To do so the welding symbol would have to include the values the detailer wants to be used. For instance, the welding symbol would list the prequalified joint in the tail, but the welding symbol would indicate a groove angle of 55 degrees, a root opening of 3/8 inch, etc. Still the skills of the fitter being what they are, will not hit the mark so the inspector verifies the actual conditions meet the detailer's "as detailed" values and adds or subtracts the "as fit-up" values. Provided the actual conditions are within the allowables, all is good on the production floor. If not, that's another thread.  

The only fly in the ointment is how does the fitter know what the nominal values of the groove angle, root opening, etc. are? There in lies the rub. Somehow that information has to be made available to the fitter, welder, and inspector. The fabricator can give everyone a copy of the applicable code or include the information with the WPS or a separate document. AWS doesn't dictate how the information flows to the production floor.

Best regards - Al

Best regards - Al