Externrefs allow you to pass variables by reference. The $ZCALL() function, on the other hand, does not accept a period before the variables in the actual list. When passing parameters by reference with an externref, you must specify an input and output MECHANISM of REFERENCE or DESCRIPTOR in the external call table.

When passing a local variable with subscripts by reference, an external call (unlike an M invocation) only passes and may only affect the value of the top-level node.