Protocol, chapter 3: [Keep] An entity's key uniquely identifies the entity within an entity set. [New] If multiple entity sets use the same entity type, the same combination of key values can appear in more than one entity set and identifies different entities, one per entity set where this key combination appears. Each of these entities has a different entity-id (link to section 4.1). [Delete] An entity can be a member of at most one entity set at any given point in time. [Keep] Entity sets provide entry points into the data model. Protocol, chapter 4.1: [Delete] Whereas entities in the Data Model are uniquely identified by their key values within an entity set, entities within a payload are identified by a durable, opaque, globally unique entity-id. [New] Whereas entities within an entity set are uniquely identified by their key values, entities are also uniquely identified by a durable, opaque, globally unique entity-id. CSDL, chapter 13: [Remove] Note: although a model may expose multiple entity sets of the same type, an entity can be a member of at most one entity set, see [OData-Protocol] .