That would be rule #4, which was the original PBR rule for it
Right, didn't they refer to it as the "Gunline" or something?
The Gunline was actually the forward portion of the Echelon Formation, made up of FF's, DD's - maybe anchored in the center by a CL (in a large fleet). The PPD's were usually reserved for the Flag ship and perhaps (if the formation was large enough) for one or two other large ships directly in front of the Flag ship.
The idea was, the gunline had the straight plasma armament, that is scary enough to tackle head on. Meanwhile the long range PPD would be firing through the gunline at any approaching ships.
The actuall PPD deployment limit is: Whatever the Flagship carries, plus an extra PPD for every 3 ships in the fleet, so: +1 for 3 ships, +2 for 6, +3 for 9, +4 for 12.
So for a full fleet formation, you'd expect to have the 4 PPD's of the DN and the rear of the echelon, then two CA's directly to it's left and right front with the other 4 PPD's (2 each). That's 8 - all usually zero'd in on a single target.