Because a daily sub permit (in Michigan at least) is only good for 90 days in the same assignment, we need to be able to lock a substitute out of an assignment if we are not able to get a full year permit on that sub. Not all assignments have a staff member attached to it, so we cannot "block from a staff member" and we do not want to block from the school - they could be a great sub, just not meet the criteria for a long term placement. This would make it easier for our district to know we have already used this sub in this assignment and they did not meet the requirements for a full year permit.