It is important to understand the difference between copying a field in the Repository and using a reference field as part of a field definition.
Copying a field creates a one-time duplication of the field characteristics. The field definition is copied as it exists at the time of creation. You are allowed to copy help text, validation rules, triggers and visualizations. When there is a change to the original field, there is no impact to the copied field.
Reference fields create a permanent link between fields. Field characteristics are permanently inherited so that when the reference field is changed, all fields referencing the changed field are also automatically updated. Once a field has been defined using a reference field, you are not be allowed to change specific inherited characteristics. You are allowed to import help text, validation rules and triggers from the reference field at any time.
A reference field does not adopt the visualizations of the referenced field.
Important Note: If you add a new rule to a reference field or change help text of a reference field, the new rule/help text is NOT automatically added to all referencing fields because the copying if these details is optional.