The dialog is accessed by clicking in the Sources region on the Columns tab of the ActiveBOMProperties panel.
The dialog is accessed only if the PCB project includes more than one PCB document.
PCB Files - this is a list of all PCB files detected in the current PCB project. Select the PCB to use then click OK. The following PCB-specific parameters will be extracted from that PCB and presented in the Columns list, ready to be included in ActiveBOM.
Center-X (Mil) - geometric center of the component
Pad-X (Mil) - location of Pad 1
Ref-X (Mil) - location of component reference point (as defined in the PCB library editor)