  The role of "Fermi arc" surface-quasiparticle states in "topological metals"
(where some Fermi surface sheets have non-zero Chern number) is examined. They
act as "Fermi-level plumbing" conduits that transfer quasiparticles among
groups of apparently-disconnected Fermi sheets with non-zero Chern numbers to
maintain equality of their chemical potentials, which is required by gauge
invariance. Fermi arcs have a chiral tangential attachment to the surface
projections of sheets of the bulk Fermi Surface: the total Chern number of each
projection equals the net chirality of arc-attachments to it. Information from
the Fermi arcs is needed to unambiguously determine the quantized part of the
anomalous Hall effect that is not determined at the bulk Fermi surface.
