The standard distribution of Apache does not provide adequate means for user
tracking, and this module provides them. What it actually does: 

+ if the user has provided the cookie header with the correct cookie-name,
   the module writes this cookie in notes with the name uid_got (accordingly,
   then it may be written to the log); 

+ if the user has arrived without the required cookie, the module issues the
   SetCookie header for him/her and writes the cookie thus issued in notes
   with the name uid_set (and this may also be written to the log); 

+ if built-in P3P support is included, the P3P header is also issued as the
   Set-Cookie header is issued. 

WWW: http://www.lexa.ru/programs/mod-uid-eng.html