The main purpose of CTQ control is selecting the factors most important to customer.
To find the CTQs, Customer Surveys, Market Analysis, Voice of Customer, Existing products, Customer Feedback are major inputs. Then prioritization tools like Pareto chart etc are used to place these CTQs in order of their impact and desiredness in final product.
The Critical to Quality (CTQ) Tree:
The critical to quality tree is used during the Design Phase of DMAIC.