In the IMS network, there are multiple call session control function nodes present and each of the nodes has different functions to perform.
I-CSCF (Interrogating-Call Session Control Function) receives SIP message from the UE and check for the authorization of UE by fetching profile from HSS and also to find out the correct S-CSCF so that it can forward the received SIP message to selected S-CSCF.
S-CSCF needs to do Multimedia Authorization what UE has requested. Based on the downloaded profile, S-CSCF takes decision to proceed further or reject the registration.
In short, I-CSCF uses interface to HSS link to find the relevant S-CSCF for the incoming request and user authorization. S-CSCF uses interface to HSS to check what multimedia related features server supports and also authentication vectors to authenticate the UE.