In PMD, the term quantity means a set of physical data that is entered in the form of a vector $$[v_1,v_2,\dots,v_N].$$

An example of a quantity can be material properties $[E,\alpha,\nu,\rho,\sigma_Y,Q_Y,\dot\varepsilon_c,\Phi].$

In PMD, quantities can depend on coordinates, time, temperature and other variables (see Dependencies of quantities and List of variables). Each component $v_n$ can be an independent function of up to four variables, i.e., $v_n=v_n(x_1,x_2,x_3,x_4).$

A quantity and its optional dependence are described by a set. The set is indicated by two key letters and an identification number that specify the type of quantity (see List of quantities). Each set is further denoted by an index number, which makes it possible to introduce several descriptions of the same physical quantity, e.g., different material properties.

Quantities are defined independently of the finite element mesh. They are assigned to individual objects, e.g., elements, surfaces, etc. In the assignment, it is necessary to reference the quantity type XY (only the first letter X is used) and the set number.