The functionality "make default" for combined states was only specified for the Creo View adapter usage but not for the view manager (see article: CS187463 ). A part or assembly file in Creo Parametric always opens in "default all" combined state. That means, that this will be the combined state that is active, when a worker is processing the file. Currently there is no chance to define a combined state that will be directly active when the file will be opened.
If you have defined different combined states that uses different appearance states, it is not possible to export all of them to STEP for example because the STEP interface currently can only handle one color set. Therefore the active appearance state will be exported. For MBD/MBE purposes it would be very helpful to be able to define a combined state as the "user"-default which will be active when opening the file. In that case it would be possible to automatically generate the correct appearance when a Windchill worker is processing the file for example.
There might be some other reasons why this would be helpful - please feel free to add some in the comments area 😉