Decide on the entities in the bi-directional word processing world and capture them in a domain model expressed as a UML class diagram (see UML tools) You are not required to have attributes or operations (methods). That is, a box with only a class name suffices for each class.
Then divide these entities into the environment and the system. Superimpose this division on to the UML-class-model-expressed domain model. You can do this superimposition by drawing a shape enclosing the environment entities and another shape enclosing the system entities. Please label each line as ENV or SYS as the case may be. Alternatively, you could just mark each domain model entity with ``ENV'', ``SYS'', or both, as the case may be.
If you have done this superimposition correctly, you should be able to see clearly which entities are in the interface.