When the crawler scans a web page, it extracts all of the links on that page for searching later. The format of the file is one record per line.
Number can have three values.
This page will never be attempted again by the robot during an automatic crawl session. However, you can still index the page by entering it directly in the Add URL form. Finally, if Number is greater than 2, then it is the numeric date of when the page was last indexed. During an automatic crawl session, the crawler will look for any addresses which haven't been updated in a while, and will re-index them.