Sender ID is a set of characters that represent the identity of a sender. For example, when you receive a regular SMS from a friend, her/his name (as stored in your contacts) is displayed as a Sender. Similarly, when a business sends an SMS to its customers, a numeric or alphabetic set of characters represents the company or brand name.
Sender ID for Promotional SMS is numeric and is decided by the operator. For example 777777
Sender ID for Transactional SMS is 6 alphabetic characters in India. For example: NOTIFY, MSGIND. For other countries, sender ID format may vary according to the rules of a particular country.