All variable names starting with the three characters rx_ are reserved for internal use in RXS. Failure is raised if such a name is used in a RXS program.
If RXS uses DB2, the IBM DB2 REXX interface is active. This interface reserves the following names for internal use: All names starting with SQL, RDI, DSN, RXSQL and QRW, plus names C1 to C100. The reservation applies only to the action block which uses func='sql'. No problem using these names is ever reported, and accordingly RXS raises no failure if they are used.
The variable names mentioned in this paper as being general orders, and mentioned for being variables for communicating the result of general orders, of course are reserved: they have a special meaning in the context in which they are used. Otherwise they are not reserved. For instance, if an action block does not access MQSeries, the variable named mq is not reserved. The only variable in RXS that have its special meaning in every context is cont. Accordingly cont cannot by used for any other purpose.
Syntactically reserved words like if, else, return may - but should not - be used as variable names. The same applies to all function and instruction names.
The index of 'RXS documentation (the PDF-document) contains references to all variables with special meaning in RXS.