Information about changed data blocks is registered in special VCT files. VCT files are stored in the C:\ProgramData\Veeam\EndpointData\CtStore folder on the Veeam Agent computer. When the backup job runs, Veeam Agent for Microsoft Windows uses VCT files to find out what data blocks have changed since the last run of the job, and copies only changed data blocks from the backed-up volume.