The number 21 recalls the 21-million cap and works as a compact, memorable prefix. Bitcoin states the field without abbreviation. The .group ending is used by holding companies and corporate groups to present the parent above individual brands, with investor relations, governance and press information in one place. Read together, 21bitcoin.group introduces an organisation with several activities. As a generic ending it works internationally.
As a brand name, the domain stands above any single product. The same name is also held on .exchange and .shop, and the three can be acquired together, so a parent company, its trading venue and its retail shop share one recognisable name. Typical uses: the corporate site of a bitcoin holding, an investor relations page, a careers portal for a group of companies, or the umbrella site of a bitcoin-focused venture studio.