Donors (Gens) have ratings similar to channels which relates to their selyn production rate and selyn draw speed tolerance, and barrier level they can control. Donor, Technical Class: TN-1 - TN-2 – TN-3
See: Rating System
The TN-1's are the First Order Donors who serve the personal Need of the QN-1 channels. The First Order were the First discovered (e.g. Rimon Farris), which is why they are termed First. Since the skills and capacity of the Firsts are more obvious, it makes sense they were the first to be identified.