Paul Fussell said this quote

“ The implicit optimism of the [field service post card] is worth noting—the way it offers no provision for transmitting news like “I have lost my left leg” or “I have been admitted into hospital wounded and do not expect to recover.” Because it provided no way of saying “I am going up the line again,” its users had to improvise. Wilfred Owen had an understanding with his mother that when he used a double line to cross out “I am being sent down to the base,” he meant he was at the front again. Close to brilliant is the way the post card allows one to admit to no state of health between being “quite” well, on the one hand, and, on the other, being so sick that one is in hospital. ”