high biological protein

[food_techie]

What is the difference between high biological protein and low biological protein? (In reference to diets, food consumption) I was asked this question and I could only think of the Atkins Diet - thinking of reports on high protein diets potential harm over long-term.
Which of these protein sources is the better in relation to a diet? What foods are from each? Is low biological protein from vegetables and high biological protein from meat, eggs etc?

[Jadies Mum]

This is a variation on the old idea of 1st and 2nd class proteins.  Animal proteins contain all amino acids = 1st class protein, vegetable protein can have very low levels or absence of some amino acids = 2nd class proteins.  This led to the idea of mixing grains with pulses in a vegetarian diet e.g. beans on toast.  The amino acids missing from the beans are present in the bread & vice versa.  Some veggie sources also are 1st class proteins, e.g. soya and Quorn (because it has egg white in it).  The body can make some amino acids for itself, but not others.  If it can't make them, they have to be eaten = essential amino acids, and animal protein contains all the essentials, hence the 1st class tag.