Item option is visible only for Dropdown and Autocomplete cells, this option provides items for those cells. Items are provided as formula results and has to be in a form of an array.
Examples of items formula:
=LOOKUP(COUNTRY, NAME) - formula returns List of country names
=Section1.A1:A10 - formula returns the range from Spreadsheet section
=Section1.B - formula returns the range column B from Table section
=ARRAY(‘one’, ‘two’, ‘three’) - formula returns an array of three provided values