In contrast with AACR2R and RDA, which begin with description of the manifestation, these rules begin with description of the work, proceed to description of the expression, then description of the manifestation, and finally description of the item. At each of these stages, the first step is to determine if this work is already described, if this expression is already described, or if this manifestation is already described. This procedure recognizes the actual process of cataloging, which does not take place in a vacuum, but is done against an existing set of cataloging records. As Ed Jones puts it:
"I think cataloging is actually two simultaneous activities: the cataloging of the object, which is primarily a manifestation-based activity, and the integration of the catalog record into the catalog, which is necessarily a work-based activity. Since one cannot begin to catalog an object until one has determined that it is new to the catalog, one must construct a provisional main entry for searching purposes before one can determine whether or not to catalog the object at all (or just add it as an additional copy to an existing record). [Sorry about all those "ones".] For the catalog search to be effective, one must determine the forms of the headings under which to search as well as the titles and variant forms. So one is tentatively cataloging the item (and performing much of the heavier intellectual work) before determining whether it is necessary to create a catalog record in the first place.
For some reason, all catalog codes are written with the assumption that cataloging is taking place in a vacuum, and the object being cataloged is the first to be entered in the catalog. In practice, the search of the catalog is the first step, and one must make a number of preliminary determinations before undertaking the catalog search:
(1) What is the title(s) under which I may find this (or something similar) in the catalog?
(2) What are the names under which I may find this (or something similar) in the catalog?
(3) What is the probable form of the headings that represent these names in the catalog?
A computer algorithm may already have searched the catalog by ISBN, etc., based on data in vendor records. If not, then a fourth determination is necessary:
(4) What are the standard identifiers under which I may find this (or something similar) in the catalog?
At this point, the cataloger searches the catalog, probably WorldCat, and tries to understand the results, which may be very complex, with dozens of candidate records for this object or related objects.
Only when a determination has been made that the object is new to the catalog, and only when its relationships with other objects in the catalog have been established, can one proceed to actually describe the object in some definitive way (though we have been examining the descriptive elements all along), identify it (perhaps within the framework of a family of objects), and provide appropriate access points.
I think the 'context of the catalog' would be very useful to include in a catalog code, not just for pragmatic purposes (because this is how we catalog) but because it tends to justify selecting the work as the Group 1 entity with which to begin the cataloging process."
Note that the approach to cataloging taken in these rules is such that data elements that apply to all expressions of a work are recorded at the work level and not repeated at the expression, title-manifestation, serial title, manifestation, or item level, and data elements that apply to all manifestations of an expression are recorded at the expression level and not repeated at the title-manifestation, manifestation, or item level, etc. This approach used to be known as degression. An important implication of this approach is that a particular data element may have to move down the work-expression-manifestation-item hierarchy over time as conditions change. For example, if all expressions of a work are illustrated in the first few years of the work's existence, the illustration statement may go at the work level, but if at some point in the future an expression of the work is published without illustrations, the illustration statement will have to move down to the expression level at that point in time. It is not really practical to practice degression in the current shared cataloging environment, but if the internet has the effect of moving us toward cataloging in a single virtual catalog, it might become more practical; it may even be possible to let computers do some of the data element movement, or at least signal to the cataloger when it might be necessary.