This attribute accepts "true" or "false".  ReadOnlyOnEdit="true" prevents users from editing field contents when existing records are opened for editing, Existing field content is visible and may be selected and copied.  Read only properties can be configured using one or more of the attributes:
  • ReadOnly
  • ReadOnlyOnEdit
  • ReadOnlyOnAdd

The following example defines an "Updated By" field as a Relationship to a record in the Users table that is by default Read Only, but editable when opened for Editing.  The field will not be editable on a new record, Brick River will set the Relationship based on the current User identity.  However, the field may be edited when an existing record is opened for editing.
<Relationship Id="UpdatedBy" Name="Last Updated By" TableId="Users" ReadOnly="true" ReadOnlyOnEdit="false" NoOverride="ReadOnly" Multiple="false" DedupIgnore="true">