Pang,
There seems to be a nuanced difference when it comes to human resources and the other areas, of which I do not fully understand. With respect to health, safety and environment, there are (US) regulations stating that you will have a plan and what the plan must cover, but the regulations are silent on HOW the plan will be implemented with any one employer. In that respect, the regulation is an EEF, the company program is an OPA.
With respect to quality, there are no regulations but there are consensus industry standards (think ISO, ANSI, etc.) that are recognized best practices but generally unenforceable; so again the company must develop it's own program. There again, the consensus standard is an EEF and the corporate policy is an OPA.
Human resource management is specifically called out in the PMBOK as an EEF. Personally, I just went with it and moved on.
A more experienced moderator may have more helpful advice here.
Harry