The two characters placed in front of the Sender ID are known as Prefix. For example, if you receive an SMS from ID-SBIBNK, Then in this message ID is the prefix. The first character of the prefix denotes the operator and the second character denotes the location from which it has been received.
See the image below.
In this particular image, The first 2 characters that is IM are the prefix of the sender ID. And the last 6 characters (PANTLS) is the sender ID of Pantaloons. You will find that whenever you receive a sender ID SMS, There will be 2 characters in front of the main sender ID. We have taken the example of Pantaloons. Here the two characters are I and M. These are automatically set by the operator delivering the message. Its main purpose is to inform about the channel used in the message delivery. Bulk SMS company has no control over this. According to TRAI (Telecom Regulatory Authority of India), the rules are, Sender ID can only be of 6 characters.