John:
To try to clear the interpretation I think you should know what it is looking for. I just read this in the ASME Y14.5M-1994 standard and will relay to you what I understand it to mean.
In your situation you are looking for the parallelism of a feature with respect to a datum in the form of a line. In section 6.6.3.1, (d) is the best representation of this. It states, "a tolerance zone defined by two parallel lines parallel to a datum plane or axis within which the line element of the surface must lie."
What I understand this to be is best explained by a simple square block where the bottom of the part is the datum to which the top surface is controlled with parallelism. Using a surface plate to locate the datum you would run an indicator across the top surface of the block in a straight line and record the maximum value and the minimum value. The distance between the highest and lowest points could not exceed your tolerance for parallelism else the part would fail this condition. It is important to know that location is in no way involved in the reporting of this measurement. Simply on the CMM you would take points on the upper surface of this block changing the x or y axis only to form a straight line and report the difference between the highest and lowest z axis value of the points. This is very similar to profile except with profile it would also control the location by defining a basic dimension for the top surface from which variation is permissible. To better state profile it would be the same slab like zone, however the zone would be centered about the axis of the basic dimension (in most cases). With parallelism the surface location could be an inch to tall from the nominal location and still pass parallelism as long as the difference between the highest and lowest are less than or equal to the allowable tolerance. To finalize this, if you were to report the distance between the B datum and each individual point of your line the difference between those two points is your actual parallelism.
I hope this wasn't an un-necessary post and you have a very clear understanding of this tolerance now. If this is not the case I apologize and will give it my all to correct any issues. If anyone finds this to be in error please correct me promptly as I do not wish to add to the problem, rather help to fix it. Thank you in advance.

Best regards,

Terry W. Prather
Metrologist


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