Primary filters, sometimes called a pre filter are plumbed before the factory filter.
Fuel passes through this relatively coarse filter first before continuing on to the vehicle’s factory installed filter, then to the high pressure pump. The filter media in a primary filter is rated at 30 micron. A micron is a unit on length equal to one millionth of a metre, or to break this down further; one micron is one thousandth of a millimetre.
So, a 30 micron filter will remove particles the size of 30 x 1,000ths of a millimetre. This may sound very small, and it is, but not small enough to protect modern high pressure fuel systems from abrasive damage. Most factory filters on modern Common Rail systems are rated at 10 micron.