A RGNDATA structure contains a header and an array of rectangles, used to store a clip list.
The rectangles in this object are sorted from top to bottom and from left to right, but they may not overlap.
The RGNDATA structure is defined in the DirectX SDK as :

typedef struct _RGNDATA {
RGNDATAHEADER rdh;
char Buffer[1];
} RGNDATA;

The RGNDATAHEADER structure present in the above structure is defined in the DirectX SDK as :

typedef struct _RGNDATAHEADER {
DWORD dwSize;
DWORD iType;
DWORD nCount;
DWORD nRgnSize;
RECT rcBound;
} RGNDATAHEADER;