Currently, as documented*, Orion only syncs named windows and the tabs in them between your devices over iCloud. (The wording suggests that it might also be possible to sync unnamed — 'temporary' — windows, but I haven't seen where how to do that might be documented yet.) This is somewhat similar to how Safari force-syncs tabs in tab groups across devices over iCloud, regardless of whether any other tabs on other devices are available as suggestions via iCloud Tabs. This makes it so that, if I understand things correctly, all tab groups are always open across all devices, or are at least displayed prominently as tab groups to open.
Whether a tab is open, part of a tab group — currently 'named window' in Orion (though that may change in future; see 1, 2) —, part of a tree of nested tabs (when using the vertical tab layout,) or synced should all be completely separate, orthogonal concerns. Tabs open on one device shouldn't be forced to open on another device, though I think this is only a problem in Safari, not Orion. Users should also be able to group or nest tabs independently of whether they're synced across devices or not. Presumably, all tabs on all devices would/should be available as suggestions of tabs a user could open another device.
* Source: https://blog.kagi.com/orion-features, under 'Tab groups.'
Use Case:
- Independent handling of which synced tabs a user has open per-device versus what tabs they have grouped or nested on each device.
Implementations in Other Browsers:
- Safari: See in-line discussion above.
Interactions with Other, Existing Features:
This suggestion affects:
- tab groups,
- nested tabs (when using the vertical tab layout,) and
- tab/named window syncing.