Files with the .SIG extension contain specific data used during the sending of messages that is added at the end, as defined and saved by the sender – a signature in the form of pure text. As such, it is a text file.

SIG files created and used by users in e-mail software usually contain a name and surname, an e-maill address and other contact information, as well as a company name. They often also contain a motto or a picture created out of ASCII characters, so-called ASCII art.