Using an English grammar analogy, entities can be thought of as nouns, while relationships can be thought of as verbs connecting two or more nouns (entities). For example:  where AHU1 supplies VAV1, both AHU1 and VAV1 are nouns (entities) and “supplies” is the verb (relationship).  Taking it further,  AHU1 (the subject) supplies (the verb/predicate) VAV1 (the object).

You can also determine the relationship direction by examining the structure of the relationship. From the context of the first entity (the subject), the relation is an “outbound” relation. While from the context of the second entity (the object) the relation is an “inbound” relation.

In the Component space, a BComponent is an entity, and BRelation is used to declare a relationship from one BComponent to another BComponent.  The from component is the subject of the relation and the to component is the object of the relation.  A relation is created by adding a dynamic BRelation slot on the object of the relationship. A “BLink” is a specialized type of relation that defines a “data-flow” relationship between a value slot of one component to a value slot of one or more other components.