[SR-Users] What does it mean when auth_check and auth_challenge flags are "0"?

[canuck15]

I am trying to decipher a kamailio.cfg file for a particular open source 
project.

There are problems with authentication and I am trying to understand why.

They use the following lines for authentication check

if (!auth_check("$fd", "subscriber", "0")) {
             auth_challenge("$fd", "0");
             exit;

The documentation only talks about using "1" and not "0".  So I am 
trying to understand what the above statement with "0" is actually 
doing.  Any help in this regard would be greatly appreciated.