Complex sentence: Most HPLC pumps have a 400 or 600 bar pressure maximum and those which are capable of operating at or above the 600 bar level are sometimes referred to as "UHPLC" systems ("Ultra High Performance Liquid Chromatography").
Split into simpler sentences: Most HPLC pumps have a 400 or 600 bar pressure maximum. HPLC systems with pumps which are capable of operating at or above the 600 bar level are sometimes referred to as "UHPLC" systems ("Ultra High Performance Liquid Chromatography").