Complex sentence: X.520 and X.521 together provide a definition of a set of attributes and object classes to be used for representing people and organizations as entries in the DIT is one of the most widely deployed white pages schema.
Split into simpler sentences: X.520 and X.521 together provide a definition of a set of attributes and object classes to be used for representing people and organizations as entries in the DIT. They are one of the most widely deployed white pages schema.