Some problem of mc_pos_control.cpp

Hi,I read the code of mc_pos_control.cpp.
Some code I can’t understand.
It’s followed:

					/* vector of desired yaw direction in XY plane, rotated by PI/2 */
					math::Vector<3> y_C(-sinf(_att_sp.yaw_body), cosf(_att_sp.yaw_body), 0.0f);

					if (fabsf(body_z(2)) > SIGMA) {
						/* desired body_x axis, orthogonal to body_z */
						body_x = y_C % body_z;

						/* keep nose to front while inverted upside down */
						if (body_z(2) < 0.0f) {
							body_x = -body_x;


					} else {
						/* desired thrust is in XY plane, set X downside to construct correct matrix,
						 * but yaw component will not be used actually */;
						body_x(2) = 1.0f;

when fabsf(body_z(2)) < SIGMA,SIGMA is 0.000001f.
body_z(2) almost equal 0,then set body_x all zero and body_x(2) 1.0f.
And the code comment upon the operation note that set X downside to construct correct matrix.
It’s mean regard x axis as downside axis instead of z axis ?
Any one give some interpretation?