An important source of built-in rules are specified in WidgetsRules.oss. In this file, you will find many useful rules for data type, decoration, trait, operation, layout, action, and module in OSS. In particular, you will find these nested OSS rules:
We covered the OSS syntax in more details in a different tutorial, but this is one of the rules:
If selectors [field=any, type=String,
match the current context values,
then apply the properties
Since MetaUI is UI library agnostic the build-in rules are layered. We have always WidgetsRules.oss in the bottom the core lib that defines generic rules and we do have WidgetRules.oss (e.g. fiori implementaiton) on top that is specific to a UI library implementation.