Ingredient lists distinguish between a named meat and a named meat meal, and the two are frequently assumed to differ in quality. The real difference is processing and water.
What rendering does
Rendering cooks animal tissue at high temperature, separating fat and driving off water, and the remaining solid is dried and ground into a dry powder.
The result is concentrated. Fresh meat is largely water, while meal contains only a small residual fraction, so an equal weight of meal carries several times the protein.
Fat is largely removed in the process and added back separately during formulation, which gives manufacturers control over the finished fat content.
Why it changes the ingredient list
Ingredients are ordered by weight before cooking. Fresh meat carries its water into that weighing and therefore ranks high.
Once the food is extruded and dried, most of that water has gone, and the contribution of the fresh meat to the finished product is much smaller than its position implies.
Meal has already lost its water before weighing, so its listed position corresponds far more closely to what it contributes to the food in the bag.
What the naming conventions signal
A named species — chicken meal, lamb meal — indicates the material came from that species specifically.
Unnamed terms such as meat meal or poultry meal indicate the source is not specified and may vary between production runs.
The distinction matters most for dogs on an elimination diet, where an unspecified source makes it impossible to know what has been excluded.
By-products and what they include
By-product meal is defined by which parts are included rather than by being inferior material. It covers organ tissue and other components excluded from muscle meat.
Organ tissue is nutritionally dense and is a normal part of what a wild canid eats, so its presence is not evidence of a poor product.
What defined terms exclude is more informative than what they include. Specific regulatory definitions rule out hair, hooves, horn and intestinal contents.
Where the real variation sits
The quality range within meal is wide, and it is determined by the freshness of the incoming material and the temperature and duration of rendering.
Excessive heat damages some amino acids and reduces their availability, which is invisible on a label and only shows up in digestibility testing.
None of this is readable from an ingredient list, which is why manufacturing standards and feeding trial substantiation tell you more about a product than the order of its first five ingredients.