There is a TSI associated with each fax. The value is assigned by the fax machine that sends the fax.
The person who sets up or maintains the fax machine is responsible for setting the TSI. The value can be any set of digits although fax machine vendors usually recommend that the user assign the facsimile telephone number as the ID.
The TSI number is also referred to as a TSI (Transmitting Subscriber Identification) number. For more information see CSI and TSI Values.
If you expect faxes from a particular fax machine or from a particular Faximum user then you can set up Incoming Fax Routing for that TSI.