Set activity_id on every item in source to the requested activity (or NULL to send the subgroup back to Ungrouped). Items whose split actually changes are flagged manual_override=True (via the apportionment writer); every matched item — including those already at the target — is additionally stamped out of distribution links, WITHOUT manual_override when the stamp is the only change. updated_count therefore means "splits changed or stopped following a distribution".

Salaries are rejected on the same grounds as :func:merge_subgroups — they're synthesised from aurd_employee_rates and apportioned via :class:AURDEmployeeCostApportionment, not stored on the expenditure items table.