A permanent dimension is a dimension that you specifically place. Permanent dimensions can display in 2 different states: modifiable and non-modifiable. You can modify a permanent dimension when the geometry that it references is selected.
If the geometry that a permanent dimension references is not selected for modification, the dimension displays at its true size and is not available for modification. This behavior eliminates crowding of dimensions when they are not needed for modification.
When dimensioning components such as doors and windows, you can select either the edge of the opening to dimension or you can select the center of the component to dimension.