Translations:PartDesign Fillet/36/en

Edge numbers are not completely stable, therefore it is advisable that you finish the main design work of your solid body before applying features like fillets and chamfers, otherwise edges could change names and filleted edges would likely become invalid. When the property  is  there is some protection from this. Because in such cases all the edges of the base object are used and there is no dependence on individual edge names.