A system can reside in a Security Domain ... for example the the may be an instance of a System in a Secret security domain and another instance in a Official domain. Equally, one could have domains of Sales , Finance and Research.
This allows to categorize the System itself.
This proposal suggests adding an element called SecurityDomain as a string.
Annotation could be 'An optional element to define the domain that the system exists in.'
Please add between Organisation and Version.