Sketcher > Dimensioning Sketcher Geometry > About Locked and Unlocked Dimensions
  
About Locked and Unlocked Dimensions
You can lock dimensions in Sketcher. Locking of dimensions allows you to avoid modifications made to the sections outside the Sketcher mode by accidentally dragging a vertex or an entity. The locked state of the dimension is maintained when you quit or re-enter Sketcher mode.
You can also set Sketcher to lock all user-defined dimensions automatically by setting the value of the sketcher_dimension_autolock configuration option to yes. Alternatively, click File > Options > Sketcher > Lock user defined dimensions.
After you set this option, all dimensions that you subsequently create or modify are locked. Dimensions created before you set automatic locking maintain their status.
 
* The locked state of a dimension is not retained if the dimension is referenced in a relation, as the relation takes priority over the locked status of the dimension.
To unlock a selected dimension, right-click and choose Unlock on the mini toolbar.
You can set the color for locked dimensions by clicking File > Options > System Appearance > Sketcher. Select a color in the Locked dimension area.