Definition of "Volume" :

Economy in  the cost of  binding. reduction in the number of undersized volumes difficult to handle, the need for splitting  over thick and composite books that may admit of such treatment and several other considerations stand in the way of counting every volume that arrives in the library as a separate book for accessioning solely on the basis that they were  stitched or cased and issued separately by the publisher. Hence, one requires a good deal of judgement in deciding what constitutes a volume for the purpose of assigning a single accession number, 

Periodical publications:

Periodical publications present a series of complications like different sequences of pagination and the existence of more than one title page in the same volume. In such situations, the question arises as to whether the volume should be separate as per the title pages and each part accessioned and bound separately.