Reserve is spelled and understood the same way in English and German, and it carries a sense of prudence: something set aside, not traded. Bitcoin reserve describes a holding kept for the long term by a company, a foundation or an individual. The word avoids speculative overtones and fits the language of treasurers, boards and auditors. On .at the name speaks to Austrian companies and investors, and German readers see it as Bitcoin-Reserve without any change.
The domain falls into the treasury category. The same name is also held on the Swiss, Dutch and UK endings, and the set can be acquired together for a provider that wants one reserve brand across several European markets. Typical projects: a treasury advisory firm for small and mid-sized companies, a reserve-management service with custody, a corporate disclosure site, or a research publication on reserve strategies.