What is the difference between an absolute and a nominal filter cartridge?

A NOMINAL 1 micron filter can trap 85 percent of particles of 1 micron and larger as it flows through whereas an ABSOLUTE 1 micron  filter traps 99.9 percent of particles larger then 1 micron. For most purposes nominal aqua filtration is fine but for high grade aqua purification an absolute-rated aqua filter is preferred.

A Micron is a unit to describe a measure of length, equal to one millionth of a metre

ABSOLUTE- One of the measurements of a micron rating of a filter…It indicates that 99 percent of any particle larger than the stated size will be  trapped within the filter.