Specifying someone as Available tells VSys that he can be given assignments on those dates and times. Negative availability - indicating someone as Not available - removes his availability from those dates and times you say he is not available.
Your organization may not track availability. You can configure VSys to not show warnings when an assignment is given to someone who has not indicated availability during the time of that assignment.
If you try to give someone an assignment during a time when they're specifically noted as "Not available", VSys will still show a warning. This is true even if availability hasn't been generally entered and the availability warnings are turned off. Enter dates and times a person is not available and VSys will tell you when you make an assignment that would be in conflict.