856 Subfield $q - Electronic format type:

Subfield $q contains an identification of the electronic format type, which is the data representation of the resource, such as text/HTML, ASCII, Postscript file, executable application, or JPEG image.  Electronic format type may be taken from enumerated lists such as registered Internet Media Types (MIME types).

The intent of specifying this element is to provide information necessary to allow people or machines to make decisions about the usability of the encoded data (what hardware and software might be required to display or execute it, for example).  The electronic format type also determines the file transfer mode, or how data are transferred through a network.  (Usually, a text file can be transferred as character data which generally restricts the text to characters in the ASCII (American National Standard Code for Information Interchange (ANSI X3.4)) character set (i.e., the basic Latin alphabet, digits 0-9, a few special characters, and most punctuation marks) and text files with characters outside of the ASCII set, or non-textual data (e.g., computer programs, image data) must be transferred using another binary mode.)