Based on input-data-size threshold

The Threshold property and the size of the input file determine whether all map instances run locally within the same Java™ Virtual Machine (JVM) or each instance of a map service runs in a separate JVM. When the input data exceeds the input-data-size threshold, each map instance runs in its own JVM and accesses the IBM Transformation Extender Translation Engine through the Sterling RMI adapter.

You configure the default input-data-size threshold on the Threshold= property in the Sterling_installation_dir\properties\translator.properties_wtx_ext.in file. The Threshold value is measured in bytes of data. The default threshold is 0, meaning that all maps run in the same JVM.