Straight only plays the incoming discrete channels, plus the subwoofer based on your bass management settings.
Surround decode will try and use all your speakers (DPL2, etc) if the incoming signal contains fewer channels, however I’m not sure if this applies to height channels on newer atmos models. If you have a 5.1 system and you get a DD 5.1 signal, the behavior is the same as straight.
Standard then builds on the behavior of Surround Decode and adds in the most mild of the cinema dsp profiles.
Im not much of a cinema dsp fan so I stick to straight or surround decode. Just depends on if you want upmixing.