The text inside the brackets — a position (1) or a file name (Budget.xlsx).
Anything before the [, kept verbatim — a directory or URL prefix, '' when there is none.
Whether book is a [n] position rather than a file name.
The sheet after the ], or null for the book-scoped [1]!Name form.
The two halves of a scope that names an external book: how the book was written, and the sheet (if any) after it.
Remarks
[1]Sheet1->{ book: '1', positional: true, sheetName: 'Sheet1' }[1]->{ book: '1', positional: true, sheetName: null }— the[1]!Nameform[Budget.xlsx]Sheet1->{ book: 'Budget.xlsx', positional: false, ... }C:\d\[Budget.xlsx]Sheet1->{ book: 'Budget.xlsx', dir: 'C:\d\', positional: false, ... }