  There are at least 2 distinct mechanisms for the formation of young massive
clusters (YMC), all of which require galactic-scale processes. One operates in
harrassed fragile galaxies, in the dense cores of low mass galaxies, at the
ends of spiral arms, or in galactic tidal shocks where transient and peculiar
high pressures make massive clouds at high densities. The result of this
process is usually only one or two YMC without the usual morphologies of local
star formation, i.e., without hierarchical structure and a continuous power law
distribution of cluster masses up to the largest mass. The other operates in
the more usual way: continuously for long periods of time in large parts of the
interstellar medium where the ambient pressure is already high as a result of
the deep potential well from background stars and other gas. This second
process makes clusters in a hierarchical fashion with size-of-sample effects,
and tends to occur in nuclear rings, merger remnants, and even the ambient ISM
of normal galaxies if the star formation rate is high enough to sample out to
the YMC range.
