Actually before UE render when calculate view-projection matrix, there is a matrix multiply:
FMatrix(
FPlane(0, 0, 1, 0),
FPlane(1, 0, 0, 0),
FPlane(0, 1, 0, 0),
FPlane(0, 0, 0, 1)
)
this make the view rotated from (x,y,z) to (z,x,y), so z is depth.