The RGNDATA structure contains a header and an array of rectangles that compose a region. The rectangles are sorted top to bottom, left to right. They do not overlap.
typedef struct _RGNDATA { 
    RGNDATAHEADER rdh; 
    char          Buffer[1]; 
} RGNDATA; 
 
  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in wingdi.h.
Regions Overview, Region Structures, RECT, RGNDATAHEADER