A Sub-Reseller is an individual/entity who does high volume purchases from you and accordingly, gets special pricing and treatment from you. He has his own chains of Sub-Resellers and an established Customer base. He adds value to the products and re-sells them to/through others.
Every Reseller can at any time look at the Orders, Customers and Sub-Resellers under him and manage only them. If you have Sub-Resellers under you, then you are considered as the parent Reseller for your Sub-Resellers.