AC = XZ shows that the horizontal pieces are congruent. This is the "S" of "ASA" as it stands for the pair of congruent sides. We are also given angle A = angle X. This is one of the "A"s in "ASA" as it stands for pair of congruent angles.
What we're missing is the other pair of angles. We must pick C and Z because they help sandwich the two sides mentioned above. ASA has the two angles surrounding the sides; or the sides are between the angles. We can't pick B and Y, unless we were doing AAS which is a similar related idea. The order matters when it comes to ASA and AAS. However, AAS is the same as SAA.
