  Let $\mathcal O$ be a holomorphy ring in a global field $K$, and $R$ a
classical maximal $\mathcal O$-order in a central simple algebra over $K$. We
study sets of lengths of factorizations of cancellative elements of $R$ into
atoms (irreducibles). In a large majority of cases there exists a transfer
homomorphism to a monoid of zero-sum sequences over a ray class group of
$\mathcal O$, which implies that all the structural finiteness results for sets
of lengths---valid for commutative Krull monoids with finite class group---hold
also true for $R$. If $\mathcal O$ is the ring of algebraic integers of a
number field $K$, we prove that in the remaining cases no such transfer
homomorphism can exist and that several invariants dealing with sets of lengths
are infinite.
