iCR "Login Reference" - explanation

Does someone have a good reference document or explanation on what the 'login reference' is with regards to obtaining an authentication token for iControl REST when using external authentication? The references I have found on DevCentral and the iCR REST and BIGIQ REST guides simply say 'get one from your system administrator'.

 

Following a post from Jason Rahm I have authentication working by using an entry of 'loginProviderName':'tmos'. However I'd rather understand what is happening here rather than leave it to nifty black magic.

 

I don't have deep RADIUS or LDAP experience so perhaps it's common knowledge that I just don't have. Appreciate any pointers.

 

Edited to add link to referenced Jason Rahm article.