World Seed is an option in the World Customisation menu in FortressCraft. It is used to 'remember' a certain world. For example, one could create a mountain tropical world, and enter the manual world seed of 7. The world could be reset to anything else. Now, if one wants, one can get that first world back by entering the same settings, and the world seed of 7. It is essentially bookmarking a certain world generation.