When editing an Option Group directly (from Administer / System Settings / Option Groups) (but not when editing the options for a custom field), I can fill in a "Grouping". There is a corresponding column "grouping" in civicrm_option_value.
The grouping isn't displayed when looking at a list of option values in the option group, or when editing a custom field on a contact.
Is there anything that makes use of the grouping within core? I'm encountering a situation in my work where it may be useful, depending on what it actually does (long list of school subjects, grouping by qualification system).