A more thorough explanation:

Feudorum Libri is a set of five books that contain Lombardic feudal law. It was published in Milan around 1152 during the reign of Henry III. Feudorum Libri was the main source of tenure law in Europe and was widely used in medieval law schools and courts in Italy, France, and Germany. For example, lawyers in these countries would refer to Feudorum Libri to understand how feudal law worked. Although it was probably known in England, it had little effect on English lawyers. However, it did influence them to study their own tenure system more critically.