In Traditional Synergy in Visual Studio you set the Target Runtime Level (TargetRuntimeLevel) and Language Compatibility Level (LanguageCompatibilityLevel) in the SYNPROJ files as MSBuild Properties.

This is normally done via the UI, however when you have a large number of projects it is inefficient to do it in this manner. Rather you'll most likely use a tool like the one described here https://synergexresourcecenter.force.com/SiteIdea?id=08732000000Cx8MAAS

When running such a tool you need to know what the valid options are for those tags.

Today you can somewhat discern this from the documentation here: https://www.synergex.com/docs/#tools/toolsChap1Compilingatraditionalsynergyroutine.htm under the "Runtime compatibility" section. However it is not clear to the end user that these values are the same ones used by TargetRuntimeLevel and LanguageCompatibilityLevel. The documentation should be clearer about this.