I will try to provide you with some information on this subject.
Each client application establish a connection with Teradata through a session. Depending the application, multiple sessions may be established in order to take advantage of the Teradata parallelism. Typically, the load applications are the ones that would open many parallel sessions.
Regarding session balancing, Teradata would take care of this for you, through the communication mechanism available between Bynet and PEs, where each new connection should be tied to a different PE. By doing this, the system keep the sessions balanced between the PEs.