The dependent variables are referred as to Transported Quantities, and their SI unit can be customized. The default mass concentration per unit of (undeformed) solid volume can be expressed in the SI unit of kilograms or moles per cubic meter. To model the transport of ions, or holes and electrons in semiconductors, the number density is also available.

By default, one species is available in the Transport in Solids interface, but multiple species can be added. The dependent variable can be retrieved by using the physics interface name (the prefix). As an example, you can plot the concentration of species c with the default physics interface name ts, so this variable reads ts.c.

The difference between material and spatial frame variables, like concentration and density, stems from the volume that is being referred to (for instance kilogram per cubic meter). A material frame variable uses the undeformed (reference) volume whereas a spatial frame variable uses the deformed (current) volume of the solid where it is transported. These are the so-called material and spatial frames, respectively. They are related through the determinant J of the deformation gradient as follows

here, cmat refers to the mass per reference unit volume, and cspa as the mass per deformed unit volume.