When people are buying or selling bows, they typically specify the draw weight at 28”. Of course, the user of the bow is more concerned with his actual draw weight at his actual draw length. The conversion is about 2# per inch of draw length at weights less than 45#, and 3# per inch of draw length at weights more than 45#. So if a person buys a 40# at 28” bow, and he is drawing 29”, his actual draw weight is about 42#.
Sometimes people buy a custom bow made specifically for their draw length, so you may see some bows marked 50# @ 30”. Not very often, though.