Enabling lip analysis in the Shell Editor Parameters sheet allows Plaque Simulator to decide which shell collimation method to use. The program will employ an analytical algorithm if the lip of the plaque is very close to circular (e.g. COMS plaques without notches).
You can suggest a preference by checking Prefer circular lip method. The analytical method is very fast, but works only for plaques with perfectly circular lips. The more general purpose ray tracing method is much slower, but provides excellent results for complex plaques with notches and/or arbitrary shapes.