Apache2 constants syntax question

Hi.

I suppose that this is a bit academical, but I'm wondering anyway.

In Apache2::Const, there are a number of constants member of the ":common" import group, 
which are (presumably) the same as their near equivalent in the ":http" import group.
For example :
Apache2::Const::FORBIDDEN ~ Apache2::Const::HTTP_FORBIDDEN
etc..

Which ones should I use ?
Is there a difference ?
If not, what was the idea in defining some of them twice with a different name ?