Randy -
I went and did a search on this and it seems that it's about how the body processes the different types of protiens - more or less efficiently. Animal proteins seem to be the best - although any protein is essential. See excerpt below.
Patient Education for University of Utah Health Sciences Center Protein Controlled Diet
Protein is an important component of many foods. Protein is needed to maintain body tissues in good health and to replace cells that are normally lost each day. There are two types of protein, high biological value (complete) protein that comes from animal sources and low biological value (incomplete) that comes from vegetables and grain products. The animal proteins are better utilized by the body than vegetable and grain proteins.
A protein-controlled diet has been planned so that the majority of protein is from animal sources. Diets containing 40 grams protein or less are usually nutritionally inadequate. Vitamin and mineral supplementation is recommended.