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Typically a classical composer's works are numbered using Opus numbers (opus being the Latin expression for “work” or “labour”) in, usually, chronological order. They allow the differentiation of one work by a single composer from all the other works of that composer and are particularly useful when seeking to differentiatiate works by a single composer that are in the same category. For instance, Joseph Haydn wrote 16 string quartets. The Opus numbers can be used to differentiate these from one another as is demonstrated by the article at https://en.wikipedia.org/wiki/List_of_string_quartets_by_Joseph_Haydn. Also, the article at https://en.wikipedia.org/wiki/Opus_number provides more details about Opus numbers.

In many cases, Opus numbers can be further sub-divided. For example, Haydn's "Prussian" string quartets are referred to as Opus 50 (often shortened to Op. 50) but each separate string quartet within the group is given a number so that the third one is referred to as "Op. 50, No. 3".

For some composers (e.g. Mozart, Bach, Buxtehude and others), musicologists have devised special catalogue numbering systems, sometimes thematic rather than chronological: Köchelverzeichnis, Bachwerkeverzeichnis and Buxtehudewerkeverzeichnis et al. See https://en.wikipedia.org/wiki/Catalogues_of_classical_compositions for additional information.

As a consequence, where they exist, the composer catalogue numbers are usually used in preference to any Opus numbers that exist. Before Ludwig von Köchel created his catalogue of Mozart’s compositions (the Köchelverzeichnis), for instance, Mozart’s compositions were numbered using Opus numbers. 

In cases where a message sender does not have opus or composer catalogue numbers as separate fields (i.e. where these are only available as part of the title), there is no compulsion to separate out such identifiers. However, companies are encouraged to provide opus or composer catalogue numbers in addition to the title (which may include opus or composer catalogue numbers):

Example in ERN-3 Syntax
<SoundRecording>
	<SoundRecordingId>...</SoundRecordingId>
	<IndirectSoundRecordingId>
		<ComposerCatalogNumber>1068</ComposerCatalogNumber>
	<IndirectSoundRecordingId>
	<ReferenceTitle>Suite No. 3 in D major (BWV 1068), No. 2: Air</ReferenceTitle>
	...
</SoundRecording>
Example in ERN-4 Syntax
<SoundRecording>
  <ResourceId>...</ResourceId>
  <WorkId>
    <ComposerCatalogNumber>1068</ComposerCatalogNumber>
  </WorkId>
  <DisplayTitleText>Suite No. 3 in D major (BWV 1068), No. 2: Air</DisplayTitleText>
  ...
</SoundRecording>
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