What is the X in X-mas?
A fixed-length string. If a string value is too long for the allocated space, it is truncated to fit; and if it is shorter, the empty space is padded, usually with space characters.
This is an allusion to Procrustes, a legendary robber of ancient Attica. He bound his victims to a bed, and if they were shorter than the bed, he stretched their limbs until they would fit; if their limbs were longer, he lopped them off.