What is the fundamental, universal prerequisite for accretion to occur, regardless of scale?

Answer

A gravitational potential well deep enough to trap incoming material

Whether it is a black hole curving spacetime or a river bend slowing water, the universal condition for accretion is the existence of a trap that captures the momentum or energy of the surrounding material.

What is the fundamental, universal prerequisite for accretion to occur, regardless of scale?
Geologyprocessplanetmassaccretion