I think that this chapter 6.2 "Entity set" is something that may differ between Redfish and Swordfish. I cant find this kind of "disclaimer" in Redfish specification.
I think also that Redfish definition of collections means that collcetions should be regarded as entity set, modelling containment/child relations only, in same way as for
entity sets on OData. (But Im not an OData expert..) Text from from OData spec: "An entity can be a member of at most one entity set".
It seems that Swordfish for some reason choses to handle this differently, by stating that no entity sets are defined explicitely. I assume that this means that
collections are NOT (always) entity set in Swordfish. Meaning that a resource may not be member
of more that one collection in Redfish, but in Swordfish it can be OK to list one resource instance (entity) in mutiple collections. (This seems also to be used
in Swordfish schemas and mockup.) In Swordfish it will be difficult to know which collation that actually conatins the resource (child/containement),
but according to gericson in below thread this is intentional, and left to each implementation. Personally I prefer the Redfish way to handle this, even if this also has some disadvantages.
See also possibly relate thread: redfishforum.com/thread/88/drives-volumes