"HEAD" and "MAIN" default tag in CVS

[David Chan]

I found that in CVS 1.11.5, there are two default auto-generated tag 
"HEAD" and "MAIN" for modules.

I found that "HEAD" tag represent the latest version of files, and when 
I select the "MAIN" tag, it seems that the same latest version of the 
files are shown.

May I know What are the difference between them?
May I know more about these two tags?
Is there any documents I could take reference to?

I tried to search over the documentation "The Cederqvist" from 
www.cvshome.org, but failed to get detail information about these two 
tags. But I am not sure if I missed any information from it, if you know 
     which section I could obtain detail information about these two 
tags, could you please tell me?

Thanks!