The 1996 American National Standard (ANSI Standard) for
Rexx is called "Programming Language - REXX", and its
number is X3.274-1996. The Standard can be ordered from
the American National Standards Institute website. The
cost is US$60 for a PDF version.

Here is the pre-publication approved final draft
of the Standard.  This draft is functionally
equivalent to the published Standard.  Mirrored from
https://www.rexxla.org/rexxlang/standards/

(The draft version appears to be free/gratis, so okay to
mirror here.)