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 > 
Options > and then click Lock user defined dimensions.
Options > 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. |