The followings are the possible logical combinations between two entities, a and b:
　　1. a ≠ b
　　　(1) resource and resource
　　　(2) resource and event
　　　(3) event and event
　　2. a = b (recursive)

□ (R) in the combination between "resource and event"

　RULE: The identifier of the resource shall be replicated into the event.

　"aRb" says that "a" stands to "b" in the relation "R", provided mathematically a description of R(a, b)
　[≡ f(x, y)]. 　In the form of "aRb", a is "a thing" and b is "a thing".

　We use "aRb" not in "relation (function)", but in relationship.　Both of a and b, therefore, are "resources" and R is an "event" when viewed in perspective of "Entity and Relationship" diagram, i.e.,
　A customer ordered a product, as in R(customer, product)--a "customer" and a "product" are entities and "order (in the form of 'verb')" is a relationship between the two entities.

　If and when viewed in perspective of "T-formed ERD", an entity has the identifier of code (or number)--so, "order (in the form of 'verb')" is an entity because it has order-NO.
　An "order", as an entity by definition, partakes of the nature of R, and R is a "logic (algorithm)", not a "thing"--it's a logical expression and the deductive inference is executed in the R. That's the reason why the identifier of resource is replicated into an event.