It could be an IBM EBCDIC code. If the Prolog processor
character set is IBM EBCDIC. From the ISO Core Standard:
In as far it would be possible that a Prolog processor supports
more decimal digits than only the latin decimal digits. Since the
standard says:
But something tells me, the ISO core standard didn’t have this
use case in mind. Rather more atoms and variables I guess.
On the same page more or less:
Edit 22.06.2022:
In the above terminology, a Prolog processor that supports
more character codes than those listed in 6.5, supports extended
characters, this holds for Prolog processors that support Unicode.
Each Unicode glyph that is not from 6.5, would be an extended
character. But then usally a Prolog processor also supports
the Unicode code point collation, which is a further ingredient
as per 6.6, i.e. the numbering of the glyphs as per Unicode.
If the later is the case then \xXXXX\ and \uXXXX say the same.
But I wonder whether a Prolog system has ever had \uXXXX
which got internally differently coded since 6.6 was not Unicode
code point numbering? Maybe there is some such Prolog system?