A more thorough explanation:

A suspicious-activity report is a form that financial institutions must fill out and send to federal authorities if they suspect that a federal crime has taken place during a monetary transaction. This form is abbreviated as SAR and was introduced in 1996, replacing two earlier forms: the criminal-referral form and the suspicious-transaction report.

Here are some examples of situations that might require a suspicious-activity report:

• A bank teller notices that a customer is making several large cash deposits in a short period of time, and the customer seems nervous or evasive when asked about the source of the funds.
• A financial advisor becomes aware that a client is investing a large amount of money in a business that seems to have no clear purpose or revenue stream.
• A credit card company receives multiple applications from individuals using the same address and Social Security number, suggesting that someone may be attempting to commit identity theft.

In each of these cases, the financial institution has reason to suspect that a federal crime may be taking place, such as money laundering, fraud, or terrorism financing. By submitting a suspicious-activity report, the institution can help law enforcement agencies investigate and prevent these crimes.