A signature file contains a small block of text that can be appended to the end of an e-mail message. A basic signature includes the sender's name and e-mail address. More advanced signatures may also include phone numbers, a company name, title, and other information.

Since signature files contain plain text, they can be edited with a text editor. However, most e-mail programs allow signature files to be created and edited within the program. Therefore, there is usually no reason to open SIG files manually.

Image format used for signs and posters created with Broderbund graphics software, such as PrintMaster and The Print Shop; can be opened with the program that created the file and possibly with other Broderbund programs or versions, but compatibility is not guaranteed.