Reference Types are a designation of types within the FrooxEngine Data Model. They are in contrast to value types in that they exist in one place in memory and are "passed around" by reference rather than by value, such as through a SyncRef<T>.
Every type in the game that is defined through a class is a reference type. Some examples of reference types include Slots, Users, Components, and Fields. Types defined through a struct are not reference types, such as the Rect type.
In general, the analogue to reference types in ProtoFlux is the object type. This can cause some confusion when using nodes that interact with FrooxEngine (such as Data Model Store) vs nodes that are contained entirely within protoflux (such as Store). A slot is an Object Ref in the Data Model, but an Object alone in ProtoFlux contexts. A string is a value in the Data Model, but an Object Field in ProtoFlux contexts.