2.2.7.4.1.2 External Cell Cache
To allow external cell references to be calculated without opening the referenced workbook, an external cell cache is stored in the file which contains cached values for cells in a sheet in an external workbook (section 2.1.10). The external cell cache contains cell values and value metadata (section 2.2.4.3) information only about the specific cells which are referenced in that sheet.
The external cell cache is composed of a collection of rows that correspond to rows in the source sheet. These rows, and their row indexes in the source sheet, are specified by a BrtExternRowHdr (section 2.4.664) record. The rows MUST be specified in order of increasing row index. Each row MUST contain one or more external cells (section 2.2.7.4.1.2.1).