Hi RUC,
I have tested the below syntax and it should work fine. One more note do not try to name the predefined condition object exactly same as existing object/class name. (Ex: in this case don't name condition object as GU)
@Select(PORTION TO PRODUCT\GU) IN @Prompt('Enter values for PRODUCT Code','A','PORTION TO PRODUCT\GU',Multi,Free)
OR
'ALL' IN @Prompt('Enter values for PRODUCT Code','A','PORTION TO PRODUCT\GU',Multi,Free)
Refer to the below Note
1806196 - Checking integrity from Information Design Tool shows an error parsing expression: ambiguous object path
Thanks
Mallik