WordWeb free version may be used indefinitely only by people who take at most two commercial flights (not more than one return flight) in any 12-month period. People who fly more than this need to purchase the Pro version if they wish to continue to use it after a 30-day trial period.
Global greenhouse gas emissions are currently around 5 tonnes of carbon dioxide per person per year, and probably need to be reduced to zero within the next decades to have a good chance of avoiding more dangerous heating. Most computer users are responsible for far more emissions than is sustainable. For example two short-medium distance return flights can be equivalent to over 1 tonne of emissions1: more than an average person can safely emit over an entire year.
If you do not qualify you must uninstall the program after the 30-day trial period or purchase WordWeb Pro. The licensing model is designed to allow relatively non-wealthy people to use the program free of charge, and to provide a small incentive for other people who fly a lot to cut down.
Whenever a user no longer meets the above requirements, and they have installed the product for more than 30 days, they must uninstall the product or purchase WordWeb Pro. Subject to the licensing terms, WordWeb Software grants you a non-transferrable and nonexclusive license to use the free WordWeb software.
There is one exception to the above: not-for-profit educational establishments may make a network installation of WordWeb for the use of their students (regardless of whether their students individually meet the licensing requirement).
1 Flights are particularly bad because of additional non-carbon emissions and cloud formation at high altitude: the short-term warming effect is estimated to be many times worse than the same CO2 emission at ground level, perhaps a factor of two worse on a twenty-year timescale, but it could be worse than this - the science is not well understood. Emissions from international aviation are also not covered by the Paris COP21 agreement, and it is extremely difficult to substitute aviation fuel with a sustainable alternative.