In MicroStation, a model stores more than just the final geometry. It stores the design intent rules that govern what will happen when geometry changes. These rules are described as Constraints.

An unrestrained rigid body in space has six degrees of freedom: three translating motions along the x, y and z axes and three rotary motions around the x, y and z axes respectively. The number of independent movements a body has, that is, how many distinct ways the body can be moved is the degree of freedom (DOF) of the body. Constraints are restrictions to these degrees of freedom of the body.

Constraint-based modeling is a technique that helps you manage the model modification process easily by implying constraints on the sketch or 2D design. These constraints enable you to preserve the original design intent even as changes are made to the model you have created. This allows for a great deal of flexibility in the design.

For example, you can assign constraints to ensure that the centers of holes always are located at twice their radius from the edges of a model.

If the holes are unconstrained, they will remain in their current location even if the base slab part of the model is modified. However, when the holes are constrained to be a particular distance from the edges, they will be relocated to honor the constraints.

Left: a model with unconstrained holes. When the base element is modified, the holes remain in their current location. | Right: model with constrained holes. When the base element is modified, the holes are relocated also to maintain the constraints.