The CABI scheme has been derived from these schemes with changes to allow for differences in subject scope and emphasis among the three databases. The enhanced database will be released in 1994. They are particularly useful for concepts which are hard to define using descriptors. For example, RR130 retrieves all records concerning Feed additives, but not those concerning Food additives. This retrieves records about resistance of cattle to pests and pathogens, but excludes items about resistance of plants to consumption by cattle.
Descriptors provide excellent retrieval capability for specific, well-defined concepts.
Broad thesaurus terms such as Soil Science or Agricultural Economics are useful descriptors only occasionally, when searching for documents which cover the subject broadly.