Transactional messages are the ones which are considered important for the users. In simple words, it can also be referred to as Service messages. Messages that are related to the services to you have already availed are placed under the category of transactional SMS.
Example:
"Dear Parents, Half yearly exam will begin on 10-12-19 Tuesday due to exam preparation & sitting arrangement holiday will be declared on 09-12-19 Monday. Thanks & Regards"
"Dear Parent, Home work for class PlayGroup is page no.- 07,08 and 15. Make sure that your ward is completing the homework regularly. Class Teacher"
"Mr., ————– Thank you for your Booking and your booking is confirmed."