The daily cap bounds the number of leads per day. The monthly cap resets on the 1st of each month. The total cap never resets: once it is reached, the campaign stops for good.
This is the confusion to avoid, and it is an expensive one. « 2,000 leads a month » is a MONTHLY cap. Setting it as the total cap would stop your campaign for good at the two thousandth lead, instead of restarting it on the first of each month.
The caps are independent: a campaign can stop on its total while the daily one is nowhere near consumed. Two rules follow. A monthly cap requires an end date, otherwise saving is refused. And a monthly cap below twenty-eight times the daily cap makes the daily one unreachable: the screen warns you without blocking you.
The schedule mostly serves to align delivery with your team's presence: a lead called within five minutes converts on a completely different scale from one handled the next day.