There is a price difference, 316 is cheaper than 904 but 904 grade is more resistant to corrosive chemicals such as the acidic by-products from burning fuels that would typically congregate in a chimney flue.
They are both approved and listed by HETAS.
The 904 grade liner is held together with a Quattrolock seal - this is what increases it’s durability. That is why 904 is advised to be installed for a Multi-fuel stove.
904 has a higher PREN Value - A PREN value is the corrosion resistance rating.