This paper presents the different ways of communication between the scientific and library word, in particular about general and specific systems of classifications (CDD and others classifications). After explanating the rules of semantic classifications, this paper shows the specific classifications for the subjects of mathematics, computer science, physics and astronomy. Furthermore, the paper deals with CDD in these subjects from the recent revision of section 510 (mathematics without theoretic computer science and physics-mathematics ….) to new edition (22nd). This paper is actually an italian draft of “Mathematics Subject Classification and related classifications in the digital world”, published in “Proceedings” of Internation Meeting “Crimea 2001” that taked place in Sudak, Ukraina, 9-17 June 2001. In this meeting was showed the new web site “La pagina delle classificazioni scientifiche” which contains the most recent documents about scientific classifications.