Take the example from the <REXX> element reference in your HEAD block. You can then use the following construct within any other element within the BODY block:
<ANYTAG NAME="=("MyREXXCode att_param")" ...>
   ...
   =("MyREXXCode other_param")
   ...
</ANYTAG>
which would be translated before it is interpreted into:
<ANYTAG NAME="ABCatt_paramXYZ" ...>
   ...
   ABCother_paramXYZ
   ...
</ANYTAG>
which doesn't make much sense, but it shows how it works in principle.

See "Sample script with REXX" for a more meaningful example.