what happens when voltage is in indeterminate region?
It is undetermined.  It could sometimes be seen as a '0' other times as a '1'. Different devices might act differently, it might vary with temperature or anything. It may even oscillate between 0 and 1.

It can't be relied upon so don't expect anything at that point.  It should be passed through as quickly as possible - a few nanoseconds so as to minimize the time that the logic level is uncertain.

If it is above or below that region the device will work as specified.