topN value in crawl

[alxsss]

 Hi,

I have read a few tutorials on running Nutch to crawl web. However, I still do 
not understand the meaning of topN variable in crawl command. In tutorials it 
is suggested to create 3 segments and fetch them with topN=1000. What if I 
create 100 segments or only one. What would be difference. My goal is to index 
urls I have in my seed file and nothing more.

Thanks.
Alex.