In addition to the standard format characters that define DTP fields, you can customize your output by specifying certain parts of a format string as callback fields. To declare a callback field, include one or more ASCII Code 88 "X" characters anywhere in the body of the format string. Like other DTP control fields, callback fields are displayed in left-to-right order, based on their location in the format string.
You can create unique callback fields by repeating the "X" character. Thus, the following format string contains two callback fields: XXddddMMMdd', 'yyyXXX. Remember, because callback fields are treated as valid fields, your application must be prepared to handle DTN_WMKEYDOWN notification messages.
When the DTP control parses the format string and encounters a callback field, it sends DTN_FORMAT and DTN_FORMATQUERY notification messages. The owner of the control must respond to these notifications to ensure that the custom information is properly displayed.