Command
A command as Telegram actually delivers one: a name, optionally the bot it is addressed to, and optionally an argument.
/start, /start@mybot, /start ABC-123 and /start@mybot ABC-123 are all the start command. The @bot suffix is not optional in a group — Telegram appends it whenever more than one bot can see the message — and the argument is how a deep link or a shared identifier enters a bot at all.