EEF Vs External Dependency.

EEFs rather are constraints for modelling your project while external dependencies are dependencies between project activities and outside activities, mostly represented by milestones in the schedule.

Examples for external dependencies are if you have to wait for another project to create a deliverable you need as input for project work (a crane to be erected by a subcontractor for building a house) or deliverables your project produces that are needed by others at a specific time (like Olympic games preparation - building a stadium).

EEFs are described in the PMBoK in length.