  Observables involving heavy quarks can be computed in perturbative QCD in two
different approximation schemes: either the quark mass dependence is fully
retained, or it is retained only where needed to regulate the collinear
singularity. The two schemes have different advantages and drawbacks. In
particular, it is known that the structure of large logarithms arising from
soft emissions is different in the two approaches. We investigate the origin of
this difference in some detail, focussing on a few specific processes. We show
that it is related to the non-commutativity of the small-mass and soft-emission
limits. Finally, we perform the resummation of soft-emission logarithms to
next-to-leading accuracy in the case of Higgs decay into a $b\bar b$ pair, in
the scheme in which the quark mass dependence is fully accounted for.
