First you have to understand the difference between EEF and OPA.
EEFs are conditions or influences (internal or external) that the project team cannot control of whereas OPAs are assets to the company. When a HR policy is affecting your scope management, it is an EEF because it is outside the control of the project even though it is an internal factor. However, when a HR policy is used as a guideline to aid the project team, say hiring an engineer, the HR policies will be an OPA.