||f forces INFLD to use a fixed-pitch font. The result is the same as if you selected the "Use Fixed Pitch Editing Font" option in the A-Shell/Windows Misc. Settings dialog, except that it only affects this field. Fixed pitch fonts look less Windows-like, but have the advantage of making the relationship between the width of the editing box and the number of characters that can be typed more predictable.
The actual font used will be the same as the standard text mode font (whose dimensions are based on the size of the main window "grid", i.e., the main window size divided by the number of rows and columns established by Tab(-5,x) and Tab(-6,x) or by SET WIDTH and SET TERM HEIGHT or the default values of 24 row and 80 columns). Because of this, the effect works best for INFLD GUI fields embedded into text-based (or hybrid text/GUI) main window displays. ||f should not be used with ||] (prevent automatic expansion of the field), since without expanding the field width by a couple of pixels on both edges to account for the internal margins of the edit control, the actual usable width of the control may be just short of the amount needed to display the specified maximum number of characters.
See ||F (Force Fixed-Pitch Font) for a variation that may work better with dialogs, especially those using the MBF_ALTPOS grid system.