Sender ID can be your organisation`s unique identity from which the message would be sent to your customers. 

For eg. when you receive an SMS from your friend, their name is displayed. Similarly, in case of Sender ID, the sender ID that you requested on Alerts/Promo panel, serves as your identity while sending messages. To know more about Sender ID, read this

Sender ID for Promotional SMS is numeric and fixed from the operator itself. For eg. 777222

Sender ID for Transactional SMS is of 6 character and alphabetic in India. For eg. VALIFY, INFINI.