A1 reference style:
=Sheet2!A1
R1C1 reference style
=Sheet2!R1C1
Note: you must enclose worksheet name in single quote if its name includes space:
='Sheet 2'!A1
To create an external cell reference that refer to a cell in a different workbook, precede the cell reference with the workbook name surrounded by square brackets, the sheet name, and an exclamation sign !, for example:
=[workbook1.xlsx]Sheet1!A1
You need to enclose workbook name (including brackets) and worksheet name in single quotation marks If the workbook name or sheet name in the reference includes one or more spaces, for example:
='[brain bell.xlsx]Sheet1'!A1
Note: Above examples will not work if the referenced workbook is closed or in a different folder. You must include the full path to the workbook reference if the referenced workbook is closed or in different location:
='D:\New Folder\[brain bell.xlsx]Sheet1'!A1
Summary
The syntax for the external cell reference to a different workbook:
'Full Path[Workbook Name]Worksheet Name'!Cell Reference
For different worksheet:
'Worksheet Name'!Cell Reference
