Chicken meal is cooked down chicken, not fresh whole or frozen that contains a certain % of water. Chicken by product meal is pretty much the same, since the cooked down parts are made from left over trimmings anyways but can include beaks and feet (I see no problem with this, some people eat chicken feet). Chicken meal doesn't contain water and can make up greater meat/protein amount then the same amount of fresh (by weight).
Corn meal is the ground corn that you cook with, made from whole corn. Corn gluten is made from the inside of the corn and is used as a protein additive - is in a fine powder form.
Basically when you're looking at a bag of food you want to see a meat or a meat meal as the first ingredient. Corn or grain products should be lower - around 3rd or 5th ingredient or none if you want a grain free food.And if that didn't make much sense or seems a little disjointed, I'm blaming the brain fog again.