This record can be used whenever you initialize materi_density in the initialization part. With density_minimum you specify the minimum density at which elements become active. Below density_minimum the elements are considered to be empty. Typically, if a material is injected into an initially empty domain you can take density_minimum to be 10 percent or so of the density at injection.
If not specified, density_minimum is taken to be 0.
The index specifies the element_group, see element_group.