Cascaded Combos on Datasheets
Most of us use Cascaded Combo boxes on normal form, idea is straight-forward, but is does not work on continuous or datasheet form. Here a database from A.D. Tejpal, Access MVP, with sample form showing how to make Cascaded Combos on Datasheets. The trick is to make combo box bound to visible field of lookup table, so data for all records will be shown in spite of combo box recordsource, and manage combo box updating in VBA code.
Scenario I used this approach – you assign users to projects, some users are gone, and you want to see them on old projects, but in combo box list you want to see actual users only.