It links the Sprwt pickup location to its record in Root Planner. The dropdown lists only Root Planner customers that were flagged as Sprwt locations (so your real customers don't appear here). You rarely need to set it manually: when a pickup location is exported for the first time, Sprwt creates the record in Root Planner and fills this field automatically. Use it only when you already created the location in Root Planner yourself or need to re-point a location. The field shows on the pickup-location page whenever the Root Planner module is active; the dropdown only lists options once your API Token is saved (otherwise it just shows "--No Customer--").