2.1.1866 Part 4 Section, FmlaTxbx (Text Formula)

a.   The standard does not define the format for the value of the FmlaTxbx element.

In Office, the value of the FmlaTxbx element shall be a string identifying a cell or a cell range in a spreadsheet.  The cell reference syntax is described in Cell References ("[ISO/IEC-29500-1] §").  Excel accepts a cell range but only considers the first cell in the range.