What is the "Credit Cap" feature listed next to each URL I'm promoting?

The Credit Cap feature allows you to control how many credits you spend each day for each URL you are promoting.  For example, suppose you have three different URLs you are promoting.  Suppose you want to spend no more than 100 credits per day on the first URL, you want the second URL to use no more than 500 credits per day and you don't want to set a maximum or "cap" for the third URL.  To accomplish this, the credit cap should be set to 100, 500 and 0 for the URLs, respectively.

The "Credit Cap Used" number indicates how many credits have been used for the given URL for the current day.