Assigning the Class Number: Once the specific subject is formulated, assigning the correct Class Number can be done with the help of the classification scheme. But this process can vary according to the scheme in use. In DDC, the Relative Index can be used to find  the relevant areas in the main schedule. The choice (tithe basic class number must be in relation to its proper context. Very often, relevant numbers front the auxiliary tables will have to be taken and added to the basic class number to form a class number co-extensive with the specific subject. Almost a similar procedure can be followed in the case of LTDC also. But here, a faceted class number will have to be constructed according to the nature of the specific subject.

In the case of CC, the use of the Facet Formula given for each  basic class will serve  as a guideline for constructing the class number. Whatever be the scheme in use, a specific subject can be classified only to the extent of specificity admitted by that scheme. In some cases it may become necessary to enumerate new sub-divisions to an existing division for accommodating the latest subjects. All  such expansions should be boded for future compliance. In the case of new  subjects not provided for in the scheme, references to current national bibliographies and new accession lists of other libraries may help to know their relative positions.  

Assigning the Book Number: After constructing  the class number, the document should be given a book number. In the case of CC, the facet formula for book number will take care of this work. In the case of the DDC and UDC, the book numbering system chosen must be used.