This macro is a flag that is used to indicate that a back buffer is lockable. This flag is used in the Flags member of the D3DMPRESENT_PARAMETERS structure.
Set this flag if the application requires the ability to lock the back buffer directly. Note that back buffers are not lockable unless the application specifies D3DPRESENTFLAG_LOCKABLE_BACKBUFFER when calling IDirect3DMobile::CreateDevice or IDirect3DMobileDevice::Reset. Lockable back buffers incur a performance cost on some graphics hardware.
Performing a lock operation (or using IDirect3DMobileDevice::CopyRects to read/write) on the lockable back buffer decreases performance on many cards. In this case, consider using textured triangles to move data to the back buffer.