NOT externalBookClosedName, which is a formula SPELLING (bracketed, and about the book's
own recorded path). This answers the different question a links pane asks — "which file is
this?" — and so it skips a source that records only a bare name: an xlPathMissing rel, or a
relative book path whose absolute companion (the xxl21 alternateUrls extension) is the entry
that actually locates the file. Sources are searched in order, so the book's own path still wins
whenever it carries a directory.
WHERE the source file is, as a path a reader can act on:
C:\dir\Book.xlsx,\server\share\Book.xlsx,https://host/Book.xlsx.nullwhen no source carries a location at all.