Metric expansion is a key feature of Big Bang cosmology and is modeled mathematically with the FLRW metric. This model is valid in the present era only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such bound objects clumps do not expand at the metric expansion rate as the universe ages, though they continue to recede from one another.
All gravity equations I know are continuous and smooth - gravity never drops to zero. Reading the above though sounds as if there was a cut-off point somewhere; bodies far enough cease to be bound entirely. It's no longer several forces overlapping and struggling, like electromagnetic and strong, struggling between binding the atom nucleus and tearing it apart. The statement makes it sound as if there was a clear-cut border between where gravity works and where metric expansion works, clusters of galaxies with their gravitational field reaching a finite distance, like free-falling snowflakes able to clump together when they meet.
Is there a border limiting range of gravity? How is the separation between gravitationally bound and independent systems explained?