Each expression variable evaluates to a numeric value with an associated unit, for example 5 cm. See “Setting Working Units” for more information. Some examples of units are distance, mass, charge and electric field. If a variable's expression is '1' then it is said to have None unit. You can add and subtract terms of the same unit. Parameters have a unit as well and they accept expressions that resolve to the same unit as them. A parameter's unit is constant. The Value column of the “Variables Control” shows the expression variable's evaluated value in working units. The tooltip of the cells in this column show the name of the unit if the unit is one of the standard types listed in the units dialog box.
In the previous figure, variable v1 has expression 5 m which evaluates to 500 cm because cm is the current length working unit. v2 has expression 10[m^2]. The square brackets allow you to define more complex units in this case such as area. You could also define v2 as 10m*m or 10*(m^2). The square bracket notation allows you to put the unit specifier right after the numeric value. The working unit for area in the above figure is m².