Walls uses a geographical reference, if one is assigned, to calculate UTM (or UPS) grid convergence and date-derived magnetic declination during compilation. You establish a geographical reference via the Geographical Reference page of a project item's Properties dialog. From there you use the built-in geographical calculator to specify the  coordinates, geodetic datum, and zone number (if UTM) for a position somewhere in the survey area. With the calculator you can verify that convergence and declination are not particularly sensitive to the exact position you choose for a typical cave survey.