ET is a skilled and disciplined approach to testing. And one of the skills exploratory testers master is the ability to manage the scope of testing so that the software is tested in a thorough and appropriate manner. Testers manage the scope of exploratory testing using a concept called a "charter."
Charters are statements of what aspects of the system are to be tested. Charters, unlike what are called "scripts" in scripted testing, do not specify how the system is to be tested, only that some aspect of the system is to be tested.