After classification work, logically the next step in the technical processing of a document is the work of assigning appropriate subject headings. These are essential for Specific Subject Entries and "See" and See also" Entries in a Dictionary Catalogue, and for Class index Entries in a Classified Catalogue. The methods employed for deriving these headings may vary from library to library. For libraries following CCC this work will be governed by the Chain Procedure. For libraries following AACR-2 either a list of Subject Headings or an indexing technique like PRECIS or POPSI will have to be used. In any case, the headings have to be chosen to correspond with the specific subject of the concerned document. If Cross Reference Entries or Subject Analytical Entries are indicated for a document, appropriate subject headings have to be derived for these also. The subject headings assigned for each document should first be noted on the Process Slip, Consistency in the use of subject headings is an essential quality of good cataloguing. This can be ensured by maintaining an Authority File in the Technical Department. An Authority File is a card file listing the headings used by a particular library and it may contain, besides subject headings, headings used in the case of difficult personal or institutional names.
The Main Entry is the entry in the catalogue which contains the maximum information about a document as permitted by the catalogue code. Most of the other entries in a catalogue are derived from the information provided in the Main Entry. The cataloguer must first prepare the Main Entry for the document. If the cards are to be handwritten the Main Entry must be written legibly. If on the other hand the cards are to be typewritten the cataloguer must write the Main Entry on the Process Slip to be given for typing.