Identification answers who the customer is; due diligence answers what they are expected to do. A retail account funded from salary that suddenly receives third-party wires from several jurisdictions has departed from its profile, and the monitoring system is designed to notice.
Requirements scale with risk. Low-risk retail accounts get automated monitoring; higher-risk categories such as a politically-exposed-person, offshore entities or cash-intensive businesses get enhanced-due-diligence with senior sign-off and periodic review.
Reviews are refreshed periodically and on trigger events, which is why long-standing customers are occasionally asked to re-confirm employment, income and source-of-funds.
Related: customer-identification-program, enhanced-due-diligence, politically-exposed-person, source-of-funds, anti-money-laundering