#[arg()] decorates a field in a #[derive(Parser)] struct and tells clap how to parse that field from the command line. Every option inside #[arg()] is a key-value pair.
long — creates a --flag style argument. If you write #[arg(long)] with no value, it uses the field name. If you write #[arg(long = "1p")] it uses that string instead. That's why we have: