Initially LLBLGen Pro just had Selfservicing, which had IEntity and IEntityCollection interfaces. (September 2003). In March 2004, Adapter was added, which uses a different paradigm: no more Save, Delete etc. methods in the entity. This meant that an entity had to have a different interface, without these methods. This interface was simply named IEntity2. Everything with a '2' is Adapter, everthing without it is either selfservicing or shared (IEntityRelation is the same for both for example, so shared). It's not called IEntityAdapter because I wanted to avoid to have names of template sets in the interface names.
