Actual Units vs Actual Regular Units?

Hello,

What is the difference between Actual Units vs Actual Regular Units. This is the first time I stumble upon Actual Regular Units and I'm unsure what it means. The hint feature isn't very helpful and neither does Google.

When I apply actual units on activity level, I usually just calculate earned units and apply them as cost and real actual hours/costs is managed separately from P6. Would the difference change between a case where the true actuals are applied in P6 and the case where only the earned are applied as actuals?

Thanks,