Hi, and welcome!
PBS Professional software is dual-licensed – it is available under an Open Source license (which is probably what you downloaded from pbspro.org), and it is also available under a Commercial license. Generally, public sector organizations (e.g., universities and research labs) that have staff with good knowledge and skills working with Open Source software, gravitate towards using the Open Source. There is no fee required for using the Open Source, however, you must adhere to the license terms (AGPLv3).
Generally, private sector organizations gravitate towards the commercially licensed software for quality assurance, for support, and for software maintenance – business generally like having “insurance” that critical issues affecting the business are addressed in a timely manner. Altair offers the commercially licensed version which has undergone a rigorous software quality process, including integration testing with all the 3rd party tools that PBS Professional uses. In addition, Altair also offers a some really nice tools (like Altair Access desktop that makes submitting jobs almost invisible) – see www.pbsworks.com for Altair’s commercial offering or send an email to email@example.com if you would like to discuss support options.
And, again, welcome to the community!