programming collective intelligence by toby segaran. How schema works?

Question

I am studying his schema but got confused how and to whom "fromid" from link is making a relation with inside urllist? or it is a misprint ? Its chapter 4 Searching and Ranking and figure 4-1. My eyes gone blind for as i have read so much but could not able to understand even the purpose of "fromid".

Here is the Book link as: http://www.uploadmb.com/dw.php?id=1447327761

Please help Thanks.