An operand is a sending operand if its contents prior to the execution of a statement may be used by the execution of the statement. An operand is a receiving operand if its contents may be changed by the execution of the statement. Operands may be referenced either explicitly or implicitly by a statement. For some statements, an operand is both a sending operand and a receiving operand. The rules for a statement specify whether operands are sending operands, receiving operands, or both when this is not clear from the context of the statement.