It’s helpful to know exactly where the data comes from if formulas refer to cells in other worksheets or workbooks. Data appearing in square brackets, [FormulasWorkbook.xlsx], come from another workbook entirely. If it is open, it appears as just the workbook name. If it is closed, the file path or URL appears before it.
Any text immediately followed by an exclamation point, like this: =Sheet1!, indicates the worksheet where the data came from. If the name of the worksheet contains spaces, it will be enclosed in single quotes.