Should screening run at onboarding, on every transaction, or both?

Detailed Explanation

Both, because they answer different questions. Onboarding and periodic customer screening ask "is this party designated?" — the population is the customer book, and the trigger is a change in either the customer record or the list. Transaction screening asks "does this payment touch a designated party?" — the population is every message, including counterparties, intermediaries and free-text payment references that never appear in the customer file. Running only one leaves an obvious gap: a clean customer can still send a payment to a sanctioned beneficiary, and a designation added after onboarding is invisible until the book is rescreened.