When you place permanent dimensions, you can lock those dimensions. When you lock a dimension, you create a constraint. A constraint (dashed blue line) appears when you select its references, as shown in the following image.
Constraint from locked dimensions
Many dimensions may correspond to the same constraint. For example, if you place a dimension between 2 walls and lock it, you have created a constraint. If you place another dimension between the same 2 walls and lock it, no additional constraint is created.
Also see Locking Permanent Dimensions.