This attribute accepts "true" or "false".  ReadOnly="true" prevents users from editing field contents, although existing 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">