Levels of Description 

Even though AACR-2 provides for the eight areas of description and for certain elements of information in each area, it is not necessary that every document catalogued should be provided with all he areas of description. Nor is it necessary to provide all the data elements in any area of description. 

The statement at 0.26 of introduction to AACR-2 makes the point clear. It states "Although the rules for description are based upon a standard - the ISBD(G), it is recognized that certain materials do not require every element of that standard. For this reason there are differences between the treatment of some materials and some others. For example, the physical description area is called the "extent of file area" in dealing with machine-readable data files. Again, the place of publication, etc., and the name of publisher, etc., "elements are not used for manuscripts, some art originals, and some three-dimensional objects and artifacts". 

AACR-2 specifies three levels of description at Rule 1.0D. According to the needs of a particular document or according to the needs of a particular library, the different levels of description will have to be used. "Libraries may choose to use the three levels of description: either a) by choosing a level of description for all items catalogued in that library or b) drawing up guidelines for the use of all three levels in one catalogue depending on the type of item being described". 

"Each of these levels is to be considered as a minimum in that, where appropriate further information may be added to the required set of data. The three levels of description allow libraries flexibility in their cataloguing policy, because they prescribe an entry that is in conformity with bibliographic standards and yet allow some materials to be described in more detail than others".