(d)
Consider the operations facet in the previously displayed classification of Food technology. The details of the general operation of “conversion” will be different for each kind of food (e.g. one can slaughter meat but not ferment it, and the reverse for barley). It is convenient for users to divide “conversion” into a series of subfacets, each being applicable to a particular kind of food. When a particular food is chosen in the “food” facet, only the “operations” subfacet relevant to it would be applicable. (Subfacets have also been called dependent or differential facets). The use of such subfacets, incidentally, reduces the risk of “false combinations” by the searcher (such as meat-ferment) which will lead to empty results sets.
(e)
A particular facet may be used in a variety of contexts in subject description. Consider how many technologies require a facet listing chemicals. Even within one subject field, chemicals may play various roles: raw materials, reagents, end products, additives and so on. What should be the content of these various facets? The obvious solution is to use the same term listing in all facets for chemicals. This solution is used a great deal in published classifications, in the form of the device “divide like”. Chemicals, for e.g. may be listed once in facet A, and any other facet involving chemicals may have the instruction “divide like A”.