Delphi User's Guide
Marked Text Fields are markers which are inserted in the text. They can be used to implement a wide range of special functions in a text processor. To name just a few:
Any group of characters within the text can be a marked text field. The maximum number of fields is 65,535. Text Control maintains the positions and numbers of the fields. It also takes care of loading, saving and clipboard operations.
A Simple Example
Adding Strings to Marked Text Fields