When you upgrade with auc, it will apply the package changes in https://sysupgrade.openwrt.org/json/v1/overview.json and auto-install owut in place of auc (and luci-app-opkg gets replaced by luci-app-packagemanager and ...). Likewise, owut also applies the package_changes, but...
The LuCI Attended Sysupgrade app does not presently do this (yet), so you usually just get errors about missing packages during the "Checking manifest" phase.
I just wrote something about that this morning, second part of Owut: OpenWrt Upgrade Tool - #535 by efahl