It is the percentage an ingredient loses to trimming or cooking. Sprwt uses it to work out how much you actually need to buy to end up with the recipe quantity: purchased quantity = recipe quantity / (1 - trim%). For example a recipe calling for 1 oz of chicken with 25% trim needs 1.33 oz purchased. This adjusted quantity is what the Inventory Shopping List (Suppliers report) shows and what sub-recipe and meal production costs are based on. A trim of 100% or more makes the quantity zero, so keep it below 100.