Apple Macintosh PowerBook 150 - Manual (Page 147)

5 Select a Macintosh application program from the directory section of the dialog box.
6 Choose a document type from the Document Type pop-up menu.
Document types are indicated by icons and four-letter codes whose meanings
are seldom obvious. For help choosing the correct document type, see the
information in the previous section, “Recommended Assignments for Some
Widely Used Application Programs.”
If you don’t know which document type to choose, try TEXT.
7 Once you’ve specified a DOS suffix, a Macintosh application program, and the
appropriate document type, click OK.
The assignment is added to the list in the control panel.
139 Exchanging Files with DOS or Windows