  Heavy flavor production is an important QCD process both in its own right and
as a key component of precision global QCD analysis. Apparent disagreements
between fixed-flavor scheme calculations of b-production rate with experimental
measurements in hadro-, lepto-, and photo-production provide new impetus to a
thorough examination of the theory and phenomenology of this process. We review
existing methods of calculation, and place them in the context of the general
PQCD framework of Collins. A distinction is drawn between scheme dependence and
implementation issues related to quark mass effects near threshold. We point
out a so far overlooked kinematic constraint on the threshold behavior, which
greatly simplifies the variable flavor number scheme. It obviates the need for
the elaborate existing prescriptions, and leads to robust predictions. It can
facilitate the study of current issues on heavy flavor production as well as
precision global QCD analysis.
