ProCalc is SHAREWARE. You are allowed to try the program out for 15 days. If, after this trial period, you do not agree that ProCalc is a professional, commercial-quality program, simply discard it, or pass it along to others. If you do indeed keep using it, a reminder dialog will appear at the end of the trial period, and therafter every 3 days, as a reminder that you are not registered. Registering also enables several features, as explained in ProCalc Help - “Registration”, available from the Help menu.
