Usage of multiple sessions by Teradata utilities like FL/ML/FE/TPT job ?

Hi All
Can someone explain working of multiple sessions used by Fastload/Multiload/TPT/FastExport job ?
I understand that max no. of sessions by a job should not be greater than total no. of amps in the system.
 
For example,
Let say one particular FastExport job is using 10 sessions. Expected rows to be exported are 100
Then how does workload is shared between multiple sessions?
Does each session exports 10 rows?
In case one session is doing all the export processing then why remaining sessions are required?
 
Thanks
Sanket