[Samba] use of passdb backend in samba active directory mode

[suresh.kandukuru at emc.com]

Dear Jeremy and samba team, when I move my nas to AD mode I see the smb.conf as shown below.
what is use of passdb backend here ?, when anybody try to authenticate to samba it uses the passdb_winbind.so and contacts the password server 10.32.125.116 to verify login/password.
attached complete smb.conf
----

realm= EMCSOHO.LOCAL
password server= 10.32.125.116
security= ADS

passdb backend= smbpasswd
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Thanks
Suresh