Sender ID identifies the name of sender. First two characters identifies the operator and circle, rest of the characters identifies the sender name.
In case of Transactional sms, the Sender id has to be 6 Alphabet (A-Z) only. Special Characters, spaces, digits are not allowed.
In case of Promotional sms, the Sender id will always be numeric, which is nothing but telemarketers registration id. The numeric sender id will be random in nature.