It represents the micron rating of a filter and it indicates that any particle larger than a specific size will be trapped within the filter. An absolute water filter is considered to be over 95% efficient and routinely filters out over 99.9% of particles which are 5 microns and greater. Although the majority of micron filters are nominal and adequate, absolute filters become more necessary when a higher grade water is needed.
Water filtration in applications that require a specific amount of debris to be removed or pathogens such as bacteria and cysts, an absolute filter will also be the safer choice. Such applications that include sterilization and post-filtration have the most use for absolute filtration.