Mike provided the key sentence here: “ASA does not have PU trailing BR with any runner starting past 1B.” Using that, I went through the FED Umpire Manual this morning and found the references I wanted.
The FED Manual states that the PU trails the BR to 1B with no runners on, or a runner on 1B only. Elsewhere it states that, with runners in scoring position, the PU stays in foul territory.
So – when the ball is on the infield and you have no runner past 1B, the PU follows the BR to 1B then moves diagonally across the field to cover a play at 3B.
If you start with runners on 2B and/or on 3B, as Mike noted, the PU moves to holding position in foul territory towards 3B. If you have a play at 3B, the PU crosses the foul line to take the call in fair territory. If you do not have a play at 3B, you move back to home, in foul territory, for a call at the plate.
WMB