The ones which are considered important for the user. 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.
These messages incorporate messages that are sent with the intension to help the client or user get information related to the services he or she is using already. Transactional SMS services do not contain any kind of marketing or promotional tone. For example, bank account holders always get SMS related to the balance detail of their bank account.
In addition, they also get an SMS whenever money is debited or credited to their bank account. So, these messages are sent purely with the purpose to inform the client about the existing services.
Since these SMS do not contain any kind of promotional or marketing information, therefore even the numbers that are included in the NDNC list get these messages. As a matter of fact, the sender details feature a format of 6 alphabets.