Time & Labor auto enrollment bulk vs. batch

Hello,

Would someone please explain the difference between Time & Labor auto enrollment bulk vs. batch mode, and under what circumstances we'd use one or the other? 

We run auto enrollment daily to pick up new hires/rehires, but it will only process new enrollments if the run control is set for bulk mode, which requires us to use a date. We don't want to use a date, we want it to pick up all employees that need to be enrolled. We're struggling to understand the meaning of the run control fields in particular the use of the date field.

Thanks