A variable is a construct that represents a non-constant value. Variables are used in parameter values. This enables changing one or more parameter values by changing a single variable value. For example, you could define an expression variable named "radius", assign it a value, and use it in a cylinder's radius parameter. Then you could define the cylinder's height parameter as "2 * radius". Changing the radius variable causes the radius and height parameters of the cylinder to update. This is called parameterizing geometry.
In addition to expression variables, AWR® AnalystTM supports variables over sets of predefined user choices, and over files. For further discussion, see here.
A variable can be used in any number of parameters.