SubTextParts have been introduced in version X13 (23.0) as a new document element to realize the new MailMerge merge block concept. But this feature can be used for many other tasks as well.
In general, a SubTextPart is a defined range of text with a Start, a Length and a Name. This name can be used to tag a SubTextPart to be protected. If the user enters this SubTextPart, the editMode property can be used to set TextControl to read only.
On leaving the SubTextPart, the subTextPartLeft event is used to switch back to edit mode.