From The x3270 Wiki

x3270 uses XLFD fonts to render its display. These fonts are an archaic technology that is slowly disappearing from Linux distributions, but they are still used by common tools like xterm.

x3270 includes its own proprietary XLFD fonts:

Font name Size in points Glyphs Encoding Line-drawing support APL Support Bold variant
3270 14 ISO 8859-1 3270cg-1a yes yes 3270bold
3270-12 12 ISO 8859-1 3270cg-1 yes 3270-12bold
3270-20 20 ISO 8859-1 3270cg-1 yes 3270-20bold
3270gt8 8 ISO 8859-1 3270cg-1 yes
3270gt12 12 ISO 8859-1 3270cg-1 yes 3270gt12-bold
3270gt16 16 ISO 8859-1 3270cg-1 yes 3270gt16-bold
3270gt24 24 ISO 8859-1 3270cg-1 yes 3270gt24-bold
3270gt32 32 ISO 8859-1 3270cg-1 yes 3270gt32-bold
3270gr 20 ISO 8859-7 (Greek) 3270cg-7 yes
3270h 14 ISO 8859-8 (Hebrew) 3270cg-8

The x3270 fonts use a proprietary encoding, and generally cannot be used with other applications.

x3270 can also use any constant-spaced XLFD font that implements a character set consistent with the host code page. For example, iso10646-1 (Unicode) fonts are generally compatible with any host code page, except for the display of DBCS characters.

See also

Wikipedia article on X11 XLFD fonts