Datum Alignment
GD&T Alignment
When performing a GD&T inspection, alignment is performed relative to the specified datums for each individual check. This is often not the current instrument alignment in the job file. This can be a surprise to folks new to GD&T, but is required in order to correctly evaluate each check. You can see this is true by evaluating the GD&T checks with the instrument not aligned to the part at all and verifying they still report the correct results.
The current instrument alignment is not used for analysis unless specifically specified with in its properties dialog.
Note that diameter checks and other checks that use the nominal features for reference will not evaluate correctly if you are not at least close in alignment with the nominal features. Therefore, an initial alignment is necessary prior to performing a final evaluation.