This is a transformation that can be applied to the substitution of content for both macros and macro parameters.
The value is taken to be rexx code to be executed. The value to be substituted should be returned in the "RXEXEC" variable.
Note that there is a limitation (which requires significant work to remove). Any macros or symbols in the rexx code will not be expanded but simply passed through. This may or may not be an issue depending on what you are trying to do.