At runtime the array field will be populated with the return data from each sub-process instance. The list element index will correspond to the activity loop index from which the sub-process instance was invoked.
For example, if the sub-process returned an integer parameter "ReturnParam" with value "X-OK" when passed "X-Ray", "PL-OK" when passed "PlasterDept" and "PH-OK" when passed "Physio", and you mapped this to an array data field "ReturnArrayField", then: