Active Directory "Principles"?

Hi everyone,

I saw in the Software Boundaries and Limits article that you should not have more than 5,000 "Active Directory Principles/Users in a SharePoint group." You should also not exceed 5,000 "security principals."

My question is: Isn't an AD principle similar in concept to a security principal? So why are they spelled differently? Is there a key difference between the two, or is this merely a typo?

Sorry I cannot link to the article directly for context; TechNet wants to verify my account first.

Thanks in advance.