File format with .REQ extension contains information uniquely identyfing the entity requesting lease of digital certificate in the form of CSR (Certificate Signing Request) data. This format is used as a part of cryptographic processing of public keys used as a means of identyfing users and machines on the internet.

REQ files are used in the process of creating digital certificates. Certification authority, after receiving such request file, uses it to generate digital certificate and issues it to the user requesting the certificate.