Where an isin identifies the security globally, a SEDOL identifies a listing line. A single company can therefore carry one ISIN and several SEDOLs, one for each market and currency in which its shares trade.
This distinction matters for index construction and for anyone reconciling a portfolio. Index providers pick a specific listing line, so a cross-listing can be in an index on one SEDOL and outside it on another.
Example: a company incorporated in Ireland has one ISIN, a SEDOL for its London line in sterling, and a separate SEDOL for its Dublin line in euro. An index that tracks the London line ignores flows on the other.
Related: isin, cusip, cross-listing, dual-listing, index-inclusion