The user can specify optional scans on the ends of a project using the PREEMPT parameter. Those scans will be shown in the summary file .sum file, but will not be sent to the telescope control and others files (.vex, crd.xx, sch.xx, .oms and .flag files. To actually utilize those scans, or for that matter, any arbitrary subset of all the input scans, specify the range of desired output scans with DOSCANS. The arguments are the scan numbers of the first and last scans as they appear in the summary file.
It is intended that DOSCANS be used mainly by the VLBA scheduler while trying to put together dynamic schedules. It is often hard to find projects that mesh together exactly without leaveing gaps. The PREEMPT=EXTRA and DOSCANS parameters are intended to let users provide productive observations to make during such otherwise unfilled gaps.
Note that DOSCANS can be used in conjunction with parameter WRAP24 to rotate the start point of full day schedules around the clock for more convenient dynamic scheduling.