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What is the difference between these 2 brushes IMM Primative and IMM Primatives H

ZBrush Usage Questions
#1

There are two IMM Primitive brushes that are in ZBrush, they seem the same.
One is named IMM Primitives and the other IMM Primitives H. There must be a difference, any one know what it is?

Thanks

#2

H primitives are not closed solids. They have an open side(s). So I guess “H” means hollow. Look at the H Cube for example. They can be joined automagically with Mesh Fusion