(This item pertains to the check boxes on the General tab of the Setup Options dialog box. You can open it through the F5 key or through the Edit | Preferences | Setup menu option.)
The Treat theo values as if stored in part coordinates check box causes PC-DMIS to treat theoretical values as if they're stored in part coordinates. This check box was added for cases where measurement routines perform loops and the alignments are changing within the looping code.
The offset parameters of the LOOP/START command take care of this issue automatically. This is for looping measurement routines that don't use the LOOP/START and LOOP/END commands, such as a WHILE - END/WHILE loop.
Consider this sample pseudo measurement routine:
ASSIGN/COUNT=4
ASSIGN/I=1
WHILE/I<4
LOOPALIGN=START/ALIGN
ALIGN/TRANSLATION OFFSET,X,50
END/ALIGN
MYCIRCLE=MEAS/CIRCLE
THEO/0,0,0
ASSIGN/I=I+1
END_WHILE/
You would expect that each time through the loop the X value would shift by 50 since the alignment is shifting by 50 each time. However, since PC-DMIS doesn't store feature data in part coordinates but in CAD and Machine coordinates, the effect is that the feature doesn't actually move in this situation even though the alignment is changing each time through the loop. This is because the CADTOPART and PARTTOMACHINE transform matrices of the alignment are both altered in the same way, the net effect is that the CADTOMACHINE transform remains unchanged. This means, by default (when this check box is not selected), PC-DMIS ends up measuring in the same spot all four times through the loop.
If PC-DMIS stored coordinates in part coordinates internally, then this would work as expected. This is where this check box comes into play. When you select this check box, PC-DMIS keeps track of the alignment that was used during the initial measurement. On subsequent measurements of a feature, it checks to see if the current alignment differs from the alignment used on the first pass. If it does, then PC-DMIS calculates the differences and shifts the feature by that difference, causing this to function as expected.
Rather than simply changing how PC-DMIS does things internally, this functionality was provided as a check box to preserve existing measurement routines.