A Suspicious Transaction Report (STR) is a formal document filed by financial institutions or other regulated entities to report transactions that appear unusual or suspicious and may indicate involvement in illegal activities, such as money laundering, fraud, or terrorist financing. STRs are a key component of Anti-Money Laundering (AML) and Counter-Terrorist Financing (CTF) compliance. 

When a transaction or pattern of transactions does not align with a customer's known financial behaviour or raises red flags, an STR is filed with relevant authorities, such as financial intelligence units (FIUs). The report provides detailed information about the transaction, the parties involved, and the reasons for suspicion. Filing STRs helps authorities investigate and combat financial crimes, ensuring that suspicious activities are scrutinised and preventing the misuse of the financial system for illicit purposes.