This sets the element group for which the new mesh should be build. You only are allowed to build a new mesh on element group 0 or element group 1. But you can also specify -all for element_group, so that all groups will be given a new mesh. Nodal unknowns, see -node_dof are mapped from the old mesh onto the new mesh. The new mesh should be sufficient fine as compared to the old mesh. The boundary of the old mesh of the element group should be specified by node_boundary records.
If this record is not specified, index element_group is 0.