Nuitrack Trial is free and has the time limit - it stops detecting body joints 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 licence key. Key is purchased here
You need license key for each sensor used with application based on Nuitrack.