These are formulas that you define yourself and you put on the pages you want in Simuland. For example, you want to make a prediction on what will bring you sell in the next round: Product Price * number of manufactured products. Rather than doing this at every turn, you define a calculation Custom performing the calculation, and if you post on the page "Products. This will make life easier for the rest of the game Follow the links "Community \ Share personal computing" for more information.
You can share your custom calculations, as you can get a personalized calculation available by someone else. It is a sharing between players from the community of Simualnd, an exchange of goods.