I contacted Wix regarding the "Nominal Micron Rating" spec they list for some filters. For example, the 57055 has a rating of 15 microns, while the 57055XP is rated at 35 microns.. I asked what filtration efficiency the micron rating is based on, and they responded that no additional information is available. I am not sure what a 15-micron rating means if it is not qualified with a percent removal. A filter could filter down to 15 microns, which sounds good, but if it only captures, say, 50% at that level of filtration, then maybe not so good. Anyone shed more light on this? Thanks.