The Foundation sees this as a contradiction to the EU’s own interoperability goals. Although XLSX is standardized as OOXML according to ISO/IEC 29500, Microsoft’s implementations often deviate from the specifications. Furthermore, features often change undocumented, which complicates compatibility with open-source software such as LibreOffice.
I don’t think you can have spreadsheets with multiple “sub sheets” (can’t think of an unambiguous name for them - basically the equivalent of browser tabs)
Pretty sure there’s no way to have graphical charts either.
The technical term is sheet. The many sheets form a workbook (your file).