Enable or disable the checkbox.
This method can be used by state implementation to set whether the checkbox can be checked/unchecked.
In some cases, the state implementation may want the checkbox to be "not changeable" from the State Editor. In this case, the checkbox can be set to disabled so that it is not changable.
The checkbox indicates whether the value entered in the "input field" should be treated as "<i>session variable name</i>" when checked, or "<i>static value</i>" when not checked.