Connie,
The EEFs and OPAs can definitely be easy to confuse. I think the simplest way to think about them is "is this thing changing my project environment or not?". For example, a regulation or law that comes externally to the project is an EEF; you have no control over how something outside your project affects your project environment. On the other hand, an internal process or technique, such as a lessons learned database, a spreadsheet, a work flow tactic, these are things that are processes set in place by your organization to complete the project. They enhance the project, rather than externally force it in a particular direction. I hope this is somewhat clarifying. You can always sit down and try and memorize some of the more common ones.
- Steven