'Nominal' values that are used to define 'base'-values in the case where input is in 'pu'-units
The parameter 'units' allows choosing between SI ('Amp Volt') and pu ('per unit') for input-parameters of components and output-variables of meters.
The default setting is 'pu'.
pu ('per unit'):
V_base = V_nom S_base = S_nom R_base = V_nom*V_nom/S_nom I_base = S_nom/V_nom
SI ('Amp Volt'):
V_base = 1 S_base = 1 R_base = 1 I_base = 1
Note that the choice between SI and pu does not affect state- and connector variables. These remain always in SI-units. It only affects input of parameter values and output variables.