Question for Curtin Syntax

Below is the example from this document, "https://maas.ubuntu.com/docs/development/preseeds.html" The first line uses "," but the next line does not. Is that just a typo? Also is this the correct forum to post this question?

late_commands:
add_repo: ["curtin", "in-target", "--", "add-apt-repository", "-y", "ppa:my/ppa"] <-- line with ","
custom: curtin in-target -- sh -c "/bin/echo -en 'Installed {{node.system_id}}' > /tmp/maas_system_id" <--line without.