What is the User Account System (UAS)?

[Phil Cox]

The more I look into this domain authentication, the more confused I
become. While reading MS KB Article Q78209, I read:

  The Netlogon service is executed to replicate the user accounts system 
  (UAS) database between a primary domain controller (PDC), a backup domain 
  controller (BDC), and member servers, and to validate logons to the
  logical domain the servers are in. 

I am assuming that the user accounts system is referring to the SAM and
info in the NetLogon share. I am reading this to be the "domain
syncronization of the SAM & other associated domain info" goes to the BDC's
(which makes sense) BUT ALSO the member servers?????? Where am I going
wrong here? Under what (if any) circumstances do memeber servers take part
in the syncronization of a domain?

Lost in a haze of gray,

Phil