username foo privilege 15 password bar ! aaa authen login default group radius local aaa authorization exec default group radius local

if radius is unavailable and you log in with user "foo" and correct password, the exec session will be privileged as exec authorization also falls back to "local".

username foo privilege 15 password bar ! aaa authen login default group radius local aaa authorization exec default group radius if-authenticated

In this case, "privilege 15" will be ignored if radius server is unavailable (due to "if-authenticated" fallback method) and your shell is unprivileged.