Formulas with links to other workbooks are displayed in two ways, depending on whether the source workbook? the one workbook that supplies data to a formula? is open or closed.
When the source is open, the link includes the workbook name in square brackets, followed by the worksheet name, an exclamation point (!), and the cells that the formula depends on. For example, the following formula adds the cells C10:C25 from the workbook named Budget.xls.
Link
=SUM([Budget.xls]Annual!C10:C25)
When the source is not open, the link includes the entire path.
Link
=SUM('C:\Reports\[Budget.xls]Annual'!C10:C25)
Note If the name of the other worksheet or workbook contains nonalphabetic characters, you must enclose the name (or the path) within single quotation marks.
Formulas that link to a defined name (name: A word or string of characters that represents a cell, range of cells, formula, or constant value. Use easy-to-understand names, such as Products, to refer to hard to understand ranges, such as Sales!C20:C30.) in another workbook use the workbook name followed by an exclamation point (!), and the name. For example, the following formula adds the cells in the range named Sales from the workbook named Budget.xls.
Link
=SUM(Budget!Sales)
Excel help > Working with data > Formulas > creating links > About linking to another workbook or program ( > What a link to another workbook looks like)