  To understand how systems of star clusters have reached their presently
observed properties constitutes a powerful probe into the physics of cluster
formation, without needing to resort to high spatial resolution observations of
individual cluster-forming regions (CFRg) in distant galaxies. In this
contribution I focus on the mass-radius relation of CFRgs, how it can be
uncovered by studying the gas expulsion phase of forming star clusters, and
what the implications are. I demonstrate that, through the tidal field impact
upon exposed star clusters, the CFRg mass-radius relation rules cluster infant
weight-loss in dependence of cluster mass. The observational constraint of a
time-invariant slope for the power-law young cluster mass function is robustly
satisfied by CFRgs with a constant mean volume density. In contrast, a constant
mean surface density would be conducive to the preferential destruction of
high-mass clusters. A purely dynamical line-of-reasoning leads therefore to a
conclusion consistent with star formation a process driven by a volume density
threshold. Developing this concept further, properties of molecular clumps and
CFRgs naturally get dissociated. This allows to understand: (i) why the star
cluster mass function is steeper than the molecular cloud (clump) mass
function; (ii) the presence of a massive star formation limit in the mass-size
space of molecular structures.
