The decoder behavior flexibly conforms to the SPEC. It uses the nonstandard features more aggressively in order to get better performance(for example, lower latency). It may change the result of the decoder and should be used carefully. Besides including all risks in normal mode, it has more risks, such as frames disorder when reference frames POC decrease in decoder order.
The decoder behavior normally conforms to the SPEC. Most behaviors conform to the SPEC but including some nonstandard features which are widely used or often used in the industry practice. This meets the request of real streams and usages, but may not 100% conform to the SPEC. It has very low risk. E.g., we will output pictures without waiting DPB being full for the lower latency, which may cause B frame disorder when there are reference frames with smaller POC after it in decoder order. And the baseline profile may be mapped to the constrained-baseline profile, but it may have problems when a real baseline stream comes with FMO or ASO.
The decoder behavior strictly conforms to the SPEC. All the decoder behaviors conform to the SPEC, not including any nonstandard behavior which is not mentioned in the SPEC.
The decoder behavior is automatically choosen.