Text and description quoted verbatim from Microsoft DirectX 7 help file, included here for convenient reference.

The D3DZBUFFERTYPE enumerated type describes depth-buffer formats for use with the D3DRENDERSTATE_ZENABLE render state.

typedef enum _D3DZBUFFERTYPE {
D3DZB_FALSE = 0,
D3DZB_TRUE = 1,
D3DZB_USEW = 2,
D3DZB_FORCE_DWORD = 0x7fffffff,
} D3DZBUFFERTYPE;

D3DZB_FALSE
Disable depth buffering.
D3DZB_TRUE
Enable z-buffering.
D3DZB_USEW
Enable w-buffering.
D3DZB_FORCE_DWORD
Forces this enumeration to compile to 32 bits in size. This value is not used.
Remarks
The D3DZB_FALSE and D3DZB_TRUE values are interchangeable with the TRUE and FALSE macro values previously used with D3DRENDERSTATE_ZENABLE.