It’s not yet a proper gltf extension with an registered definition. It’s for importing and reimporting metadata related to real world machines.
The extras field is for metadata about the machine, and the extensions field is to support complex rigging that can’t be accomplished with single parent hierarchies (custom assets provided by a plugin). Otherwise I would use the normal bone structure supported by gltf into control rig / skeletal mesh.
USD is a better fit but I don’t believe it’s supported by interchange just yet.