Smart APIs use annotations to denote string values used to represent an entity or property. For example, when showing property name(s) in your client's user interface, the literal property name is not always a good choice as it cannot contain spaces and has to conform to PascalCase requirements.
These annotations are accessible from the annotations metadata document, which is referenced from the main metadata document.
For example, the metadata document for the SampleService Smart API used for the OData+ resource examples is:
https://sam.spclouddave.com/SampleService/Sppid/v2/$metadata
The annotations metadata document is:
https://sam.spclouddave.com/SampleService/Sppid/v2/Annotations/$metadata