"Oil purifier" is a rather generic term, but it usually refers to a system that does a more thorough job of cleaning and conditioning oil than a conventional filter system is designed for. A purifier would remove moisture, entrained gases, very small size particulates, etc. Purifier are often completely independent systems that can periodically be connected to a reservoir to purify the oil supply without taking the machine off-line. One "theory" behind using this type of oil purifier is that if you have many machines with oil systems in a particular installation, you can save cost on each machine by equipping it with just a basic oil filtration system and then perform the more complex filtering functions periodically using a shared purifier machine.