The default since Enterprise PL/I V3R2 is ARCH(2). You should not use a lower value for the ARCH option since Enterprise PL/I requires Language Environment 1.3 (and later) and Language Environment 1.3 requires machines that support ARCH(2).
Note that if you specify an ARCH level that is greater than the lowest level of any machine on which your code runs, then it is likely that your code will abend with a specification exception on those machines.
If you specify a value of 0 or 1, the compiler will reset the value to 2.