Control is an itype. The contents of a control will depend upon the operation being used by it's parent Iteration.
For sql operations the control will be a legal SQL query. Regarding SQL, if there are no results generated, then the body will not be evaluated, so in general SQL control statements are select statements.
For repeat operations, the control must evaluate to a number.
For while and while_not operations, the control refers to an input which may exist or not.