Exchange 03 Standard Email Store Limit

Okay I have looked this up and seen similar questions to what I am asking, but there is still a question that I am still left with asking. I know that 75GB is the limit. But....

Is that limit both the edb AND stm added together and is the sum total I want to avoid.

Or

Is that limit something else I have to look at to see if the 75GB has been reached?

Also for my knowledge edb is email that has already been accessed by the users and the stm is email that is waiting to be check by users?

Thanks