CityJSON uses JSON Schemas to document and validate the data model, schemas should be seen as basically validating the syntax of a JSON document.
A CityJSON Extension is a JSON file that allows us to document how the core data model of CityJSON may be extended, and to validate CityJSON files containing new objects and/or attributes. This is conceptually akin to the Application Domain Extensions (ADEs) in CityGML; see Section 10.13 of the official CityGML documentation.
Registry of current Extensions
Linear Cell Complex
An extension that allows topological information of a city model to be stored as a Combinatorial Map based Linear Cell Complex
developers: 3D geoinformation TU Delft