This page is devoted to the internals of the API package; in this particular case (since gt-api mosty consists of interfaces) there is not a lot of interesting design elements to discuss.
The gt-api interfaces for Style are a straight extension of the gt-opengis interfaces. This does come with a drawback; we need to ask you to be careful of thread safety.
The Filter classes in gt-api are deprecated; and have been so since GeoTools 2.3. We are having trouble removing all the existing test cases that depend on these old Filter definitions.