The default since Enterprise PL/I V3R6 is ARCH(5).
Note that Enterprise PL/I requires Language Environment 1.10 (and later) and Language Environment 1.10 requires machines that support ARCH(5).
If you specify an ARCH level that is greater than the lowest level of any machine on which your code runs, your code might cause a program abend with a specification exception on those machines.
If you specify an ARCH value less than 5, the compiler resets it to 5.