Casual vacancy vs intermittent vacancy

Hello Friends,

Can somebody tell me the difference between Intermittent Vacancy and Casual Vacancy in the office of Director?.  The act has neither defined Intermittent Vacancy nor Casual vacancy.

In Companies rules regarding WOMAN DIRECTOR and INDEPENDENT DIRECTOR, the act has used the term Intermittent Vacancy where as in some sections the act has used Casual Vacancy. I want to know the basic difference between these two different kinds of vacancies. 

Please enlighten me!

Regards.