Any character you can type
Text mode's alphabet was whatever had been drawn as a 5x7 bitmap in the source —
about a hundred characters, and nothing outside them. Paste a row of symbols
into the custom glyphs field and the ones nobody had drawn were dropped in
silence.
Now anything you type or paste is drawn, rendered through a font on the machine:
stars, notes, flowers, arrows, box drawing, dingbats, scripts — whatever the
system has a face for. Each character is looked up across several fonts, so a
symbol missing from one is found in another instead of coming out as an empty
box, and it is rasterised once per size and cached.
Mixing is just typing: A♪★b✿ is four ordinary letters and symbols in one set,
ordered darkest to brightest like any other glyph set.
Emoji
They work, in colour, wherever the platform has a colour emoji font — Windows
and macOS have one as standard, and most Linux installs can add one. A colour
emoji keeps its own colours and ignores the ink setting, which is the whole
point of drawing one.
Also
The glyph set list gains font (type anything), which forces the font path
even for plain letters. You rarely need it: typing a character no built-in set
has switches to the font on its own.