In the family editor, you can create labeled dimensions and flex the dimensions to create parametric relationships in the family, then lock the dimensions to enforce these constraints in the finished design.
If the labeled dimension is not locked, you can move a reference plane or reference line that is already constrained by the length parameter, and flex the family. If the dimension is locked, then you cannot flex the family by moving a reference plane or reference line. To flex the family with locked dimensions, you must change the parameter values in the Family Types dialog.