What is the purpose of "upstream" tracking branches, e.g. when you see "Your branch is up to date with 'origin/main'"?

A A tracking branch establishes a link between a local branch and a remote branch, allowing commands like "git pull" and "git push" to know which remote branch to sync with by default, and enabling Git to report how far ahead/behind the branches are B Upstream branches are a deprecated feature with no modern use C Tracking branches only apply to the "main" branch, never others D Setting an upstream branch deletes the local branch