This function has no effect unless the RegisterUserHook hook has been set.

The value to be used should be obtained by calling WinAddAtom with the handle of the system atom manager, and subtracting DTYP_ATOM_OFFSET from the result.

WinAddAtom is guaranteed to return values in the range 0xC000 to 0xFFFF.

When a data type is defined using this function, a definition for the corresponding pointer data type is automatically established.