Ketryx allows users to define the subsystem project versions that you would like to reference in the system project. 

In the Settings for the Sub-system version, set the referenced project version for the subsystem, to point to that particular version of the used service. Otherwise, by default, the latest version would be chosen. You could also set a fixed version for the "parent" System project, but usually the Latest version should be fine.

Pleaser watch this Loom for a demonstration of this feature.