Custom Furniture is a content pack framework mod by Platonymous which allows for adding new furniture items. Unlike Category:Content Patcher, which overwrites existing furniture with new sprites but does not change functionality, CF items are independently generated and do not affect vanilla assets. Custom furniture items are added to Robin's shop and the Furniture Catalog, and generally (but not always) function the same as vanilla furniture items. Unlike items, they are not assigned IDs, and thus cannot be spawned in with CJB Item Spawner.
Custom Furniture mods, like many content packs, will contain a manifest.json (with general information about the mod, such as name, author and update key), a content.json (with the specifics of the mod) and at least one image file (PNG format) containing the sprite(s) of the furniture item(s).
Note that there is a bug in the current (0.12.0) where CF items will also show up in Pierre's festival lists, with no prices (so they may be obtained for free).