L.2.13.3.1 Cell B2

The formula expressed in cell B2 (cell B2 is the c

SUM([1]Sheet1!$A$1:$A$3)

element whose r=“B2”) is this:

The external reference to another workbook in this case is tokenized to [1]. The value inside the brackets is a 1-based index to the externalReferences collection in the workbook part.