There are two types of sms,
1. Promotional sms
2. Transactional sms.
Promotional sms are used for company promotion or any other advertising purpose. As per the TRAI law,
Promotional sms is only for business / product promotion. Messages can not be sent on DND enabled mobile numbers.
Message sending time: 9 AM to 9 PM.
Transactional sms is used for any type of transactional notification,
for example: You can send sms to all your students to alert them for Exams.
After payment, you can send sms for payment confirmation .
Also you can send any notification for holidays and examination.