XLSX is awkward for scripts, databases and import tools that expect plain text. Feather keeps column types and loads fast in pandas and R. XLSX is at home in spreadsheets with formulas, formatting and several sheets; FEATHER is the usual choice for quickly saving a data frame between Python and R sessions.
One XLSX can produce several FEATHER files; when it does, they are offered together as a ZIP download.
Each column gets one type chosen from all of its values: a column is numeric only if every non-empty cell is a number, and anything mixed, or a value such as a ZIP code with a leading zero, stays text. A workbook with several sheets gives one file per sheet.