PO ACTIVITY_DATE vs LAST_DTTM_UPDATE

What are the main differences between the PO ACTIVITY_DATE and the LAST_DTTM_UPDATE fields on the PO_HDR table?  When would one field be updated and not the other?  So far, I only see that receiving will update the ACTIVITY_DATE and not the LAST_DTTM_UPDATE on the PO.

PO updated online - both fields are updated.

PO Cancelled – both fields are updated.

PO Dispatched - neither field is updated but the PO_DISPATCHED table is updated.

PO Changed – both fields are updated.

Receiving – ACTIVITY_DATE is updated and the LAST_DTTM_UPDATE field is NOT updated.