Nuitrack Trial is free and has time limit. It stops working after running for three minutes, so you need to restart it. Nuitrack Trial is intended for demo and evaluation purposes only.
Nuitrack Pro is for commercial applications. It allows to develop and to sell applications based on Nuitrack.
There are two types of Nuitrack Pro licenses: online subscription and perpetual (with unlimited period of validity).You can upgrade Nuitrack Trial to Nuitrack Pro by entering a license key. The license is purchased here