These are used on Gould systems for non-base registers syms.
However, the following values are not the values used by Gould; they are the values which GNU has been documenting for these values for a long time, without actually checking what Gould uses. I include these values only because perhaps some someone actually did something with the GNU information (I hope not, why GNU knowingly assigned wrong values to these in the header file is a complete mystery to me).
240 0xf0 N_NBTEXT ?? 242 0xf2 N_NBDATA ?? 244 0xf4 N_NBBSS ?? 246 0xf6 N_NBSTS ?? 248 0xf8 N_NBLCS ??