  Planar spin transport in disordered ultrathin magnetic bilayers comprising a
ferromagnet and a normal metal (typically used for spin pumping, spin Seebeck
and spin-orbit torque experiments) is investigated theoretically. Using a
tight-binding model that treats extrinsic spin Hall effect, spin swapping and
spin relaxation on equal footing, we show that the nature of spin-orbit coupled
transport dramatically depends on ratio between the layers thickness $d$ and
the mean free path $\lambda$. While spin Hall effect dominates in the diffusive
limit ($d\gg\lambda$), spin swapping dominates in Knudsen regime
($d\lesssim\lambda$). A remarkable consequence is that the symmetry of the
spin-orbit torque exerted on the ferromagnet is entirely different in these two
regimes.
