1. This is a holding in a suit that prevents the opponents from cashing more than a certain number of tricks in the suit. An ace or void (in a suit contract) constitutes a first- round control, a king or singleton (in a suit contract) constitutes a second-round control.
To contemplate a grand slam, a partnership will need first-round control in every suit. To contemplate a small slam, it will need first-round control in at least three suits and first- or second-round control in the fourth.
This hand has first-round control in diamonds, second-round control in each major and third-round control in clubs. If 6♠ has any hope of being a viable contract, partner will need first- or second-round control of clubs and at least two first-round controls in total.
2. In some systems, notably the Blue Club, aces and kings have specific numerical values: two for an ace and one for a king. In this case, for example, a hand with one ace and one king or with three kings would have three controls.