How does EAP-SIM module decide whether read keys from file or calculate by itself?

Hi, all After I studied the EAP-SIM and EAP-AKA parts, I have some doubts and questions: 1.I think the function eap_sim_sendchallenge is to calculate keys and send to authenticator, then authenticator forward it to the client. 2.I think the function process_eap_sim_challenge is to check the response and to decide weather to response with EAP-Success. 3.As said in 1, the eap_sim_sendchallenge can calculate keys itself, and also the rlm_sim_file module makes it can read keys from file. How to decide which method to use? Waiting for your replies, thank you! Best wishes! Lanny Zhao