I raised this on the groff list.

Because eqn uses Greek letters as mathematical symbols rather than
linguistic text, I think it is reasonable for it to explicitly access,
without loss of generality, upright and slanted symbol faces for "Alpha"
and "alpha", respectively.  I claim that by defining these keywords
(really macros, in the eqn language, except in GNU eqn, out of
laziness or optimization their defaults are set up in the lexer[2][3]),
they acquire _semantics_.  For practical purposes, the lowercase Greek
letters become slanted.

In fact, I am tempted to rename the "character types" in GNU eqn from
"letter" and "digit" (the latter being unused) to "slanted" and
"upright", respectively.  (Recognizing the old names for backward
compatibility, yadda yadda yadda.)

From my working copy:

+(The
+.RB \[lq] digit \[rq]
+type is not used by default,
+but is available for customization.)
+.\" XXX: How would you actually customize it, though?  There doesn't
+.\" seem to be a means of replacing the font associated with a type.
+.\" Is the "digit" type just cruft?

"digit" seems to really mean "non-letter".  Maybe "nonletter" would be a better name.  See the only test of its value.

https://git.savannah.gnu.org/cgit/groff.git/tree/src/preproc/eqn/text.cpp?h=1.23.0.rc4#n496