Encounters or entire missions can take place within a holodeck that provides a simulated environment. The mainstream holodeck technology used is sufficiently advanced and immersive that all Planetfall mechanics and consequences work more-or-less identically to in the 'real world' within this simulated environment, with the only tweaks being minor ad-hoc adjustments needed to bridge the gap where necessary.
In general, Planetfall missions and encounters that take place within a holodeck will use immersive "Neurophysiological Interface" (NPI) technology, meaning that all normal game mechanics carry across into the virtual world with almost no modification. In particular:
While other holodecks systems that work differently to the 'regular Planetfall rules' conceivably exist in setting (e.g. video games where everyone has flying skills), for gameplay purposes we suggest avoiding anything that vastly changes the mechanics without very good reason. If you find yourself writing a mission that really requires this, consider whether it would make more sense running as its own LARP system rather than as Planetfall! If in doubt, speak to the Refs.
Holodeck encounters and missions can cover more or less anything you want, so long as a suitable amount of input data would exist in the setting from which the holodeck program could be made: recordings, scans, archive data etc.
Note: The 'extrapolator software' that processes the input to form holodeck encounters is analogous to OC present-day neural network technology that can take example phrases and spin out entire stories or artworks based on them - only more advanced. Importantly, this is not true intelligence in any way - much like the OC neural networks that get marketed as 'AI', it's just fancy pattern matching and extrapolation, and nothing more.