OptionalsheetName: string | null
Omitted for the book-scoped [1]!Name form, which shows as C:\dir\[Book.xlsx].
This is the form Excel's own formula bar shows for a book it cannot see, which is the state every
book is in until a resolver opens one (DESIGN-EXTERNAL-LINKS §4B). The caller quotes it —
sheetNameToString does, since a path carries separators no sheet name may.
The directory is the first source that carries one: a book Excel recorded by a RELATIVE path
(NamesSource.xlsx, a sibling of the host) keeps its absolute location in the xxl21
alternateUrls companion, and Excel's formula bar shows that location, not the bare name — a
bare name is the OPEN spelling (externalBookOpenName).
Excel's source-CLOSED spelling of a scope into this book:
C:\dir\[Book.xlsx]Sheet1, unquoted.