"Calling" is the action of God in summoning people to sanctified responsibilities.
If God calls a person to a duty or responsibility, the called person has a commission to commit himself to it. In addition to this authorization, there is the element of compulsion. If God calls a person, it is never in doubt that a person is under obligation to obey the summons.
The general calling is the call of God to believe in Christ as Lord and Savior and to follow their instruction for living. Unique specific callings are God's leading a person into specific duties and responsibilities in life.