The timer clearly has no knowledge of what filtration you're using. We designed the split grade mode to use ch.1 for the soft exposure and ch.2 for the hard; if used together with Ilford filters (grades 00 and 5 respectively) and a typical white-light source the notional "grade" indication on the timer will be approximately correct. Other filters, colour enlargers etc. will differ, sometimes substantially. However the basic premise will hold i.e. in split grade mode, adjusting ch.1 mostly affects the overall exposure whereas adjusting ch.2 mostly affects contrast.