Understanding Defensive Personnel Packages: 50 vs. 40 Personnel

Question
What are coaches referring to when they talk about 50 personnel or 40 personnel?
For example "it's like a 4-3 only with 50 personnel"


Answer
    Usually what they mean is how many Defensive line men are in the game, example would be 4 or 5 linemen package. If there is 5 then this means that maybe the defensive backfield would shrink from 4 to 3, because of the defensive line change.
hope that helps, Coach Noel