exact use of "security event logging" ?

[mgar]

On FGT we have three options to set the logging in a policy-

1. Disable/No log - nothing is logged

2. log All - All traffic matching the security policy is logged

3. Security event logging - ??

first two are clear, but the 3rd one normally should log only when anything is matching to security profiles (if enabled in the policy) but normal traffic session logs should not be logged, But we notice in fortigate v6.4 this 3rd option is also logging all the traffic session logs as well. 

can someone please give some clarification on same or a relevant document explaining the same.