Translations:Creating a simple part with Part WB/88/en

Face 2 is the right side of the original face 2, the left side of former face 2 is now face 8. The cylindrical part was face 9, but is now face 7. FreeCAD reassigns the numbering and the order is not necessarily preserved. The total face count in the first model is 10, in the version with the cylindrical face piercing the side face, the total face count is 11. So obviously face numbering has to change when the so called topology changes. This probably feels like a minute detail, but turns out to be quite important in parametric 3d cad. Imagine that you have used the cylindrical face as reference for another feature, it used to be called face 9, but is now called face 8. The reference to the intended cylindrical surface is lost. Since FreeCAD, at least in currently released versions does not keep track of the intended face, it only keeps track of the numbered face, a model breaks when a reference is made to a face that later is renumbered. This is called TNP, Topological Naming Problem.