You can define top-level parameters which can be references with $ref="#/parameters/$parameter"
You can define top-level responses which can be references with $ref="#/responses/$response"

I find it usefull to add @SWG\Response(ref="#/responses/todo") to the operations when i'm starting out with writting the swagger documentation.
As it bypasses the "@SWG\Get() requires at least one @SWG\Response()" error and you'll get a nice list of the available api calls in swagger-ui.

Then later, a search for '#/responses/todo' will reveal the operations I haven't documented yet.

And although definitions are generally used for model-level schema's' they can be used for smaller things as well.
Like a @SWG\Schema, @SWG\Property or @SWG\Items that is uses multiple times.