When the start of run is given synchronously to several boards connected in daisy chain it is necessary to compensate for the delay in the propagation of the start (or stop) signal through the chain. This register sets the delay (expressed in units of trigger clock cycles - 10ns) between the arrival of the start signal at the input of the board (either S-IN or TRG-IN) and the actual start of run.
The delay is usually zero for the last board in the chain and rises going backwards along the chain.