Hi Emi,
I am by no means an expert, but what I’ve found out so far is, that define.xml does not “help” the validation of xpt files.
Rather, supplying a define-file results in P21 running cross-reference checks between xpt files and define. For an overview of the checks, look in the validation report, sheet “Rules” and filter the Category column for “Meatadata”, that seems to provide an overview.
What also happens when you supply a define-file is, that in the “Dataset Summary” sheet, Class and Label information for the xpt files is taken from the define-file. Which can be a bit confusing, since this can differ from how P21 classifies (and hence validates) data.
Jenny