An elements will be refined depending on the size of a solution variable. Typically the solution variableresidue is used, but others can be chosen via control_mesh_refine_locally_unknown.
The percentage of elements which will be refined is specified by percentage. Typically percentage is 10 or so.
This local mesh refinement is only available for -bar2, -bar3, -tria3, -tria6, -tet4 and -tet10 elements.
See the rules for old and new at control_mesh_refine_globally.
See also control_mesh_refine_locally_geometry.