The defining criterion is manual operation: a sieve or riddle worked by hand belongs in 9604 regardless of its mesh material or frame construction. Motorized or mechanically driven separating or sifting equipment is classified in the chapter covering industrial machinery and mechanical appliances, not here.

Yes, provided they are hand-operated. Both kitchen flour sifters and hand-operated laboratory test sieves fall within heading 9604. The end-use sector — culinary versus scientific — does not change the classification, though it may affect applicable standards or documentation requirements for commercial buyers.