The number refers to the hardware chipset or firmware revision inside high-quality multiplexer cables (often referred to as "Titanium cables" or "EDIC + Titanium combo interfaces"). The 43021 driver is the result of reverse-engineering the latest manufacturer handshake sequences, allowing third-party tools to maintain compatibility with dealer-level software.
The version you mentioned (1.61 with 43021 drivers) is frequently available as an instant digital download rather than a physical product: ecm titanium 1.61 with 43021 driver %5BNEW%5D
If you’d like, I can: