What is difference between Solid and X-Solid in Bricscad?

The Bricscad Platinum "X-Solids" environment captures the history of solids as they are created, allowing parametric editing of these solids at any time. Any primitive parameter can be easily changed, even after the primitives have been combined into a composite shape. Primitives within composites can be moved, copied, rotated, deleted and more while still within the composite solid. Fillets and chamfers can be resized or removed, slices can be moved or deleted, and Boolean operations can be undone one step at a time or all at once.