What is the difference between ad-hoc and contractual appointment in india ?

Respected Sir / Madam, What is the difference between ad-hoc and contractual appointment in same lecturer’s post in state govt. polytechnic setup ? Are the ad-hocs more vulnerable to regularisation of service than contractuals ? Lecturer’s post is class – I officer. The ad-hoc lecturers were selected through personal contact and put to the posts as ad-hoc. But, contractuals were selected through wide advt. in newspapers, called by interview letters after screening of applications, interviewed by interview board. Both were posted in permanent vacant posts. Both have renewable tenures after every 06 months. As the name is ad-hoc, are they regularisable automatically after some periods ? Contractuals are terminate able anytime ? Please do valued advise.