The STB classification scheme is enumerative not synthetic in structure. There are no facilities for number building whatsoever. The schedules consist of a hierarchically arranged list of topics in all facets of the whole of science and technology and are therefore extremely long. About 30,000 individual topics are listed. The draw back is that as it is not possible to synthesise numbers for compound subjects, these have to be placed at the class mark for the prime facet unless specific, with added class-marks assigned as appropriate, unless provision is made in the schedules for that particular compound.
Notation consists of basically two letters and two numbers.
Because of its broad base it is generally possible to keep it brief and therefore memorable. The simple four digit notation will also fit conveniently on the spines of most books. Brevity is also helped by the fact that the notation was never designed to be expressive, that is it does not reflect the subject hierarchy in the way that, say, the notation of the Dewey Decimal Classification does. There is also a hidden decimal principle in the numeric part of the notation which helps to make it hospitable. This is very convenient to classifiers and schedule revisers, but can be confusing to the public as the decimal point is invisible and readers often expect JF302 to file as an integer, not as JF30.2.