These terms refer to the efficiency of the element. Absolute is a high efficiency element that utilizes depth filtration (multi-layers). Nominal is a standard efficiency element that utilizes surface filtration (single layer, i.e. wire mesh or paper). ISO 16889: 1999 defines a depth filtration high efficiency (absolute) element as having a beta ratio of ßx>= 75, as determined by a multi-pass test. No industry standard exists for determining the efficiency of a surface filtration standard (nominal) element, therefore it does not have a beta rating.