KSTACK_COUNT

The KSTACK_COUNT seems to have no purpose outside the KPROCESS:

typedef union _KSTACK_COUNT {           // 6.1 only
    LONG volatile Value;
    struct {
        /*  bit fields, see below  */
    };
} KSTACK_COUNT;

Windows 8 shifts the volatility to where the structure appears in the KPROCESS as the StackCount member:

typedef union _KSTACK_COUNT {           // 6.2 and higher
    LONG Value;
    struct {
        /*  bit fields, see below  */
    };
} KSTACK_COUNT;

Whatever the details of the packaging, the point seems to be entirely to have collected two previously existing members into bit fields:

Mask Definition Versions Remarks
0x00000007
ULONG volatile State : 3;
6.1 only previously UCHAR at 0x66 and 0x96
ULONG State : 3;
6.2 and higher  
0xFFFFFFF8
ULONG StackCount : 29;
6.1 and higher previously ULONG_PTR at 0x6C and 0xA0