Link this item to an employee distribution, or opt it out.

'all' / 'engineering' link it (an unavailable scope is accepted — the link persists and the sync no-ops until the aggregate exists; the UI's disabled state is guidance, not an invariant). 'none' is the explicit opt-out — uniform for individually-linked items and subgroup-derived members; an opted-out item is not recaptured by a future subgroup link. Never writes NULL (the follow-subgroup default) — once toggled, the item's state is its own. Documented permanence, not an oversight.

Opting out also zeroes the row's R&D (see :func:_zero_item_rd_for_unlink): the derived split goes with the link. Editing the split is the path that keeps the numbers — it opts the item out on its own.