When you are mapping your fields in Bedrock you will find a column for setting a Read Only flag on your fields. By default, this is set to No but you can mark it as Yes for a given field or fields. By marking this field "Yes" Bedrock will never be allowed to write or update information on that particular field.
When should a field be marked as Read Only?
While placing a system at the top of the list will make it the System of Record. Therefore, when this field has a value it can't be overwritten by any of the other fields or objects below it. If you NEVER want it to be populated, even when there's no value and the other systems and objects do have values, you would set this field to Read Only preventing from any updates to this field taking place. Some might call this making the field a One-Way sync.