Vertex Group: Use a chosen vertex group to control the position of instances.

Using Vertex Groups on Multiple Objects: Using vertex groups when you're using a collection as the surface input can be a bit confusing at first. The way this system works is by using a shared vertex group. Two different objects in your scene can have vertex groups with the same name. When you do this, you have a shared vertex group. Now, select one of the objects with the vertex group, and choose it in the addon panel. Now the vertex groups on both objects will be used as a culling mask in the scatter system.