A checkbox is in this state if it was selected, but then disabled. The state cannot be accurately and unambiguously determined.
|JScript - 3.0|
|Internet Explorer - 4.0|
|Property/method value type:||Boolean primitive|
Checkboxes may be enabled and disabled. If you have a checkbox that was enabled and then checked, and then if it is subsequently disabled, this flag property is set to true which indicates that the current state of the checkbox is indeterminate.