The pressure gauge is after the pre-filters so that it can indicate to you when they need changing. If you note the pressure when you fit new filters, you will notice as the filters age the pressure on the gauge will gradually start to drop. When this happens you will need to change your filters for optimum performance.
Flush valve - this is to allow you to flush the surface of your RO membrane with water to remove the debris build-up. Basically, every so often (once a week or two minutes before each processing session) turn the flush valve and allow the unit to simply flush water down the drain. Doing this will prolong the life of your RO membrane by up to two years.