Parameters define the size, shape, position, material, and other information about an element in the model. Parametric modeling refers to the relationships among all elements in a project that enable the coordination and change management that Revit provides. These relationships are created either automatically by the software or by you as you work. Here are some examples:

A door is a fixed dimension from an adjacent partition wall. If you move the wall, the door retains this relationship to the wall.
The edge of a floor or roof is related to the exterior wall so that, when you move the exterior wall, the floor or roof remains connected.
Rebar is spaced equally across a given element. If you change the length of an element, Revit maintains equal spacing among elements.

Temporary dimensions