An SR-21 may be used in different states for different reasons such as for verification of insurance, a crash report or just as a form number.
For example in Florida they refer to a SR-21 for insurance verification. A person can receive a suspension of their license for a SR-21 denial. This denial is a result of a Florida driver providing the Department of Public Safety with proof of motor vehicle insurance coverage upon demand and that coverage being denied by the named insurance company. The information is sent to the company via print list and is required to be confirmed or denied and returned to the department for processing. The DPS issues a suspension as soon as they get a denied message from the insurance company.
In Georgia a SR-21 is a notice of Liability insurance form. In GA this is an insurance carrier form and should be filed in an accident if there was Liability insurance in effect on the date of the accident. So proof of insurance to the State if you were involved in an accident.
The SR-21 is most widely known for the state of Indiana's SR-21, Certificate of Compliance. Here it is a form that must be filed by your auto insurance agent after an accident. Filing this form certifies you met the state's financial responsibility requirements at the time of the accident.
An SR 21 form in Indiana may also be referred to as the Indiana Operator Proof of Insurance Report. Since January 1, 2006 the driver of any vehicle involved in an auto accident resulting in injury or death or total property damage of $1000 or more must make a report on an SR 21 form to the Indiana Bureau of Motor Vehicles within 10 days of the accident.
In Indiana the SR-21 was known until the end of 2005 as the Indiana Operator's Vehicle Crash Report. For collisions in Indiana occurring after December 31, 2005, the operator of a vehicle involved in a motor vehicle accident is required to send the form to the Bureau of Motor Vehicles. The Bureau revised the form to better identify who is actually insured on the form. The new form is titled "Indiana Operator's Proof of Insurance/Crash Report" and is State Form 52441 but still referred to as a SR-21.
Since this document is proving that the vehicle operator was insured this form must be signed by an authorized insurance representative (your agent). You can download the form from the Indiana Bureau of Motor Vehicles (BMV) if you are in Indiana.
In most states if they have a SR-21 it is to verify that you have at least the state required auto insurance in place however to find out how your state uses this term you can contact your Department of Motor Vehicles.