Dr. Ranganathan revolutionized classification in the early 20th century by proposing a dynamic and nuanced system, which could easily adapt to new subjects and incorporate more details and sub categories. One of his ideas was the Alien-Penumbral-Umbral-Penumbral-Alien, or APUPA system/pattern. It classifies material, based upon, how closely they relate to a specific topic. He created the APUPA pattern as a method of classifying books and documents based on their relevance to the searcher.
As per this method, documents are classified into three categories, that is, Alien, Penumbral and Umbral. Here, an Umbral document implies a relevant document, which is of main interest to the members of a library. A Penumbral document is meant for the marginal interests of the readers. This type is partially relevant and in some way or the other related to an Umbral document. An Alien document is non- relevant and thus, not required by the
reader. So, we can recognize the pattern which indicates that every helpful sequence of
book is Alien- Penumbral- Umbral- Penumbral- Alien, i.e., the APUPA arrangement.
The APUPA pattern, thus, arranges the most relevant documents at the center, documents
of marginal relevance on both sides of the relevant document and the totally disconnected
documents are far from the center. This is the best way to maintain filiatory sequence. The
filiatory sequence implies the placement of all the entities of a field of knowledge in a
definite sequence, in one line, according to the degree of their mutual affinities. A helpful
sequence therefore, is said to be the one which follows the APUPA pattern. APUPA
patterns are dynamic. Any book or other resource within a classification scheme can be an
Umbral source and any resource can also be Penumbral or Alien, depending upon the
subject.
As mentioned above, this sequence puts the most relevant records in the centre. The records
which are connected with it are placed before and after it; and those which are totally
disconnected are put at a distance from the Umbral. It is this kind of an APUPA
arrangement, which gives the reader great satisfaction. If it manages to do so, then it is said
to be in full conformity with all the Five Laws of Library Science.
Example: If a user is seeking a book on 'growing oranges', that book is the Umbral source. Books about 'packing and transporting oranges'; a related topic but not exactly the same are Penumbral sources. Books about setting up industry for bottling orange juice would be Alien, or unrelated sources.