A DTWAIN_STRING is an array of characters.  It is used for character data that is passed between the application and DTWAIN.  

A DTWAIN_STRING will always match the character set type of the DTWAIN DLL that is being used.  For example, if the application is using DTWAIN32U.DLL, the DTWAIN_STRING will be a Unicode (16-bit character) string, similarly DTWAIN32.DLL will use ANSI (8-bit character) strings.

If your application needs to explicitly create ANSI or Unicode strings, regardless of the DTWAIN DLL that's being used, the following types can be used:

The DTWAIN_ANSISTRING is an array of ANSI (8-bit) characters.  It is used for ASCII/ANSI character data that is passed between the application and DTWAIN.  

The DTWAIN_WIDESTRING is an array of Unicode (16-bit) characters.  It is used for Unicode character data that is passed between the application and DTWAIN.