Have you have noticed that your rtw flat front trousers are looser on one side of the front? If it is, then that tells you that the trousers are not dressed. On pleated trousers its not as noticeable because of the fullness in the pleats. Rtw trousers are not dressed and sometimes called center dress. If this is a custom garment the cutter will ask you "on which side you dress". If its m2m the person measuring should ask you the question. If you are ordering m2m on line, then you must specify this. you might use the traditional cutting room term "dress left cut the right", or the reverse dress right cut the left". Trouser patterns are cut with dress allowance on both sides. In the cutting room the cloth is doubled and the two fronts are cut at the same time. Then the excess cloth is cut off from the side where it is not needed. The diagram shows a left dress. the dotted lines show the cloth to be cut off. in this diagram, the customer is facing you.