Sketcher > Dimensioning Sketcher Geometry > Automatically Locking User-defined Dimensions
  
Automatically Locking User-defined Dimensions
You can specify whether you want to automatically lock the dimension that you create or modify by setting the value of the sketcher_dimension_autolock configuration option to yes. Alternatively, click File > Options > Sketcher and then click Lock user defined dimensions.
After you specify that user-defined dimensions are to be locked, all dimensions that you subsequently create or modify are locked automatically. 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.