Oleum concentration description What is the meaning of "free" $\ce{SO3}$ and "combined" $\ce{SO3}$ in the oleum sample concentration description?

Also what are these maximum and minimum values of % labelling in oleum sample?
I think the term combined "$\ce{SO3}$" is nothing but $\ce{SO3}$ of $\ce{(SO3 + H2O)}$ and free $\ce{SO3}$ means that is $\ce{SO3}$ in oleum sample.

And value of % leveling of oleum is from (100% to 122.5%).