Flooding recalculates the pour area and recreates all clearances for the current obstacles within the pour outline, observing clearance rules. Hatching refills (with hatch lines) existing pour polygons for the current session; it does not recalculate the pour area. Each time you open a design file, you must flood or hatch the design; this information is not saved. In most cases, you can simply Hatch. Use Flood if you make changes to the copper pour polygon that could create clearance violations or if you change clearance rules.& ^