Nuitrack Trial is free and has the 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: annual (which is valid for 1 year) 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
You need a license key for each sensor used with an application based on Nuitrack.