The term "handle" originated from CB radio culture in the 1970s — your handle was your on-air identity. When social media emerged, Twitter adopted the @ symbol and the handle terminology. Other platforms followed, though some (like GitHub) prefer "username" and others (like gaming platforms) use "gamertag" or "screen name."
Reserved handles include all the same categories as reserved usernames: system terms, brand names, celebrities, government agencies, and premium short handles. The @admin handle is blocked on Twitter for the same reason the "admin" username is blocked on GitHub. The underlying protection goals are identical.
For developers building apps, the handle vs. username distinction is purely cosmetic. Your reservation system needs to block the same terms regardless of what you call user identifiers. The API endpoint might be /check-username or /validate-handle, but the blocklist logic is the same.
One practical difference: platforms with visible @ symbols (Twitter, Instagram) face more impersonation risk. When handles appear prominently in the UI, users are more likely to trust official-looking ones like @support or @verified. Platforms with less visible usernames (like many SaaS apps) still need reservations, but the phishing risk is somewhat lower.