What does (did?) it mean when aclocal exits with code 63?

[Zack Weinberg]

One of Autoconf's regression tests includes:

# If Autoconf is too old, or the user has turned caching off, skip:
AT_CHECK([aclocal || { ret=$?; test $ret -eq 63 && ret=77; exit $ret; }],
         [], [], [ignore])

The effect is to skip the rest of the test if aclocal's exit status is 63.
This code was written in 2006, I can't find any documentation on what
it means for aclocal to exit with code 63, and I also can't find any
code in the current automake git repo that looks like it would make
aclocal exit with code 63.  (*automake* can exit with code 63 under
some circumstances, but it really looks like aclocal never will.)

Does anyone remember what situation this might have been trying to detect?

Thanks,
zw