A MAR archive is a utility created by PCE developer Hampa Hug. The purpose is to combine features of MacBinary and TAR, encapsulating the Resource Fork, Data Fork, and Finder data of a classic Macintosh file.
MAR files always contain exactly one item, either a single file or a folder and its contents (including subfolders). By convention archives that contain a file get the extension .MAC (this is basically equivalent to a MacBinary file) and archives that contain a folder get the extension .MAR. It is the same file format, though.
MAR file will begin with the hex values