default_eap_type in ttls configuraion in file eap.conf

Hi! I have a question regarding to the default_eap_type setting for ttls configuration in file eap.conf. From TTLS protocol, it is not necessary to do authentication in the tunnel and it is the user who decides and initiates which eap type to use inside tunnel. What the default_eap_type is used for? Thanks! Gina Zhang