Attenuation field indicates the apparent attenuation expected from your yeast. Attenuation varies with yeast strain. There are several factors that affect attenuation including yeast strain, mash temp, ingredient choices, fermentation temp...etc.
So when creating a recipe you want to use your best guess. If you have already brewed and you know what you final gravity is you can enter your Original Gravity reading in the Analysis Tab. Doing this will automatically calculate your attenuation.