Whenever a file is created it is assigned a unique serial number or "CRC" (Cyclic Redundancy Check). If the file is modified in any way it is given a new CRC. Using Validator you can create your own CRC or 'Signature' for a file or group of files. This Signature is saved as a .SFV file (Simple File Verifier). When the SFV is opened by using the program or double clicking it gets the CRC of the files and compares it to the original. If any of the files have been changed it will determine which one has as well as any that are missing.