These four are all duplex stainless steels. When sorted by alloy content and corrosion resistance from low to high: 2101 < 2304 < 2205 < 2507. What they have in common is that their strength is twice that of 316L, and they possess excellent resistance to stress corrosion cracking. To put it simply: 2101 is an "economical lightweight option", 2304 is an "entry-level alternative", 2205 is an "all-round mainstay", and 2507 is a "super premium grade". The core differences lie in their Pitting Resistance Equivalent Number (PREN) and applicable scenarios.