Right now extras are just a 'flat' list of checkboxes.
(yes, you can define which extras go with which bikes - see here about extras compatibility)
i.e. if you may offer 5 extras for a given bike - your customers can check some, none or all of them. No intelligence about grouping or either / or decisions.
Choice Groups for extras will put them into groups, with defined behaviours.
The above will build on all the existing extras features like setting if an extra is default selected and if it is mandatory.
further reading here:
https://brm2.bikerentalmanager.com/support/solutions/articles/42000078699-what-are-extras-in-brm-