Coring out is a technique where you remove material from a plastic part, leaving distinct walls and ribs which provide enough strength and mating surfaces for other parts in the assembly. It is necessary to make the part moldable and also saves cost and weight. Key points are leaving ribs in the right location and size to maintain strength, particularly in bending, as well as leaving surfaces and features which interface with the other parts in the assembly the part is used in. These might be bosses, spring clips, etc.