The letter being revered is a way of indicating if the beam went from the back to front (posterior to anterior) PA or front to back, AP. X-rays are effectively

The letter being revered is a way of indicating if the beam went from the back to front (posterior to anterior) PA or front to back, AP. X-rays are effectively shadows and things further from the X-ray plate get more magnified. Large structures like the chest benefit from being done a certain way, like having the heart near the plate. That way it’s less magnified. PA or AP is often a thing you see on the corner of films, indicating the technique. The film also used to be processed in a film processor, and it was very easy to accidentally flip. This is another reason we have conventions.