Question:
How does OptiSurface Designer (OSD) know what area I have paid for?

Answer:
Once you carry out an operation for a particular field topographic survey, you need to save the OptiSurface Designer file (.osd) that contains the survey and results of the operation you just performed. This effectively transfers the OptiSurface Credits from an encrypted licence file into the OptiSurface Designer file (.osd) you just saved.  Then you can repeat that operation as many times as you want using the same OptiSurface Designer file (.osd). You can open the same OptiSurface Designer file (.osd) a week or even years later and redo the same operation without having to pay again. It is stored in the OptiSurface Designer file (.osd) that you have paid for that survey. You can even email that file to someone else who has OptiSurface Designer and they can open the file and repeat the operation without charge.

However, if you survey that same areas, say 5 years later, and import the survey into OptiSurface Designer, you will have to pay again as it is a new survey.