OptiSurface Designer is a pay-as-you-go software (per acre). There are no long term contracts of commitments on your part but when you carry out certain operations (e.g. calculate landform design) you consume OptiSurface Credits.

Once you pay for an operation for a particular field topographic survey, you save the OptiSurface Designer file (.osd) that contains the survey and results of the operation you just performed. This effectively transfers the credits into your OptiSurface Designer file (.osd).  Then you can repeat that operation as many times as you want. You can also open that OptiSurface Designer file (.osd) a week or even years later and do 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.