The scavenger energy feedback loop uses the BPMs in LI19 and just the first four BPMs in the scavenger extraction line (EP01). There is a non-zero dispersion (needed for measuring the energy) only at the EP01 BPMs. In fact the energy measurement is mainly determined by units 185 and 186 just before the energy collimator. BPMs further down-stream are not used to avoid the reduction of capture range that would occur due to the small energy acceptance of the collimator.
Note that it is the kicker and Lambertson magnets which bend the beam to cause the dispersion at the above BPMs and hence set the energy scale for the loop. As part of setup, it may be necessary to adjust their strengths relative to that of the main bend string to get good orbits everywhere in EP01.
Having measured the energy with the BPMs, the scavenger energy feedback loop then controls the beam by adjusting the phases of sectors 17 and 18. This is accomplished with solid state (fast) phase shifters in series with the phase shifter of the sub-booster. It does this control a little indirectly via the feedforward hardware. Namely it writes to six DACs in the feedforward hardware (these are seen as EP01 AMPL units 3-8 in the control system). The feedforward hardware then uses those numbers and its measurements of the beam intensities to determine the actual phase shifter settings.