Why the result of cv::affine3d(4x4) times cv::vec3d(3 dimensions vector) is still CV::Vec3d?

Sorry, i can not show any more information about pose due to my power.
But could you tell me if pose is a 3x3 matrix, how does it contains these tow matrixs?
I mean if pose is a 4x4 matrix, then the left upper matrix is a 3x3 rotation and behind it is a 3x1 translation vector.
maybe pose is a 3x4 matrix?