constrainedModulePrefixes refers to the modules that you want to enforce, e.g. [ "Data" ]

allowedModulePrefixes should be [] unless there are special module prefixes within your own repository that you allow constrainedModulePrefixes modules to import from.

OnlyImportOutside.rule
    { constrainedModulePrefixes = [ "Data" ]
    , allowedModulePrefixes = []
    }