Frame dividers
For transoms and mullions in a frame, the offset is measured from the edge of the frame to the centre point of the transom/mullion. This is easy to demonstrate if we set our Drawing Board frame ruler settings to Centre (under /Settings/Organisation/DrawingBoard):
Sash dividers
For transoms and mullions in a sash (including door rails and muntins), the offset is measured as the sightline of the adjacent glass or panel. This is easy to demonstrate by switching on the sightline rulers within the Drawing Board (in the Settings panel in the bottom right):
