The Freeware Edition is free for personal and commercial use but is limited to monitoring only up to three sensors (SNMP or packet sniffer based only).
If you want to monitor more sensors or want to use Netflow based monitoring you must purchase one of the commercial editions.
The commercial editions offer the following added features (depending on the edition):
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.