Scatter node is used for intial instances generation over connected surfaces.

A surface node has to be connected to the node and a scattered object has to be selected for scattering.
Scatter node allows to setup a collision sphere (relative position and size) to determine scattered object size which is used for instances collisions in collision nodes and for boundaries nodes.

Replace material option replaces scattered object original material to a another one.
Object color sets color for scatter preview in viewport and for use by fstorm object color texture.
Objects number sets general scattered instances number.

There are to ways to set objects number - fixed and dencity.

Max limit limits maximum number of instances scattered by density way to avoid system hang in case of too many instances result.
Changing seed allows to get different scattered instances positions.
Use surface normals controlls instances orientation to surface normals directions and acts like gravity power.

Mask allows to controll instaces dencity via texture.
Invert, threshold and randomness give extra mapping controll.