Synonyms in polymer chemistry: constitutional repeating unit (CRU) and structural repeating unit (SRU)

A regular single-strand polymer consists of repeating units with two terminal groups at either end. The IUPAC Gold Book [1] defines a constitutional repeating unit (CRU) in a polymer as “the smallest unit the repetition of which constitutes a regular macromolecule, a regular oligomer molecule, a regular block or a regular chain.” The term structural repeating unit (SRU) is used synonymously for CRU [2]. A constitutional unit is an atom or group of atoms (with pendant atoms or groups, if any) comprising a part of the essential structure of a macromolecule, an oligomer molecule, a block or a chain [3].

References
[1] http://goldbook.iupac.org/C01286.html
[2] J. Kahovec, R. B. Fox and K. Hatada (Working Group): Nomenclature of Regular Single-Stranded Organic Polymers (IUPAC Recommendations 2002). Pure Appl. Chem. 2002, 74 (10), pp. 1921-1956. PDF-version.
[3] A. D. Jenkins, P. Kratochvíl, R. F. T. Stepto and U. W. Suter: Glossary of basic terms in polymer science (IUPAC Recommendations 1996). Pure Appl. Chem. 1996, 68 (12), pp. 2287-2311. PDF-version.