  I consider variations in the definitions, at next-to-leading order (NLO) and
at next-to-next-to leading order (NNLO), of a General-Mass Variable Flavour
Number Scheme (GM-VFNS) for heavy flavour structure functions. I also define a
new "optimal" scheme choice improving the smoothness of the transition from one
flavour number to the next. I investigate the variation of the structure
function for a fixed set of parton distribution functions (PDFs) and also the
change in the PDFs when a new MSTW2008-type global fit to data is performed for
each GM-VFNS. At NLO the parton distributions, and predictions using them at
hadron colliders, can vary by 2-3% from the mean value. At NNLO there is far
more stability with varying GM-VFNS definition, and changes in PDFs and
predictions are less than 1%, with most variation at very small x values.
Hence, mass-scheme variation is an additional and significant source of
uncertainty when considering parton distributions, but as with all perturbative
uncertainties, it diminishes quickly as higher orders are included.
