Starting with Juju 3.1, a ‘base’ replaces the older notion of ‘series’.
In Juju, a base is a way to identify a particular operating system (OS) image for a Juju machine.
This can be done via the name of the OS followed by the @ symbol and the channel of the OS that you want to target, specified in terms of <track> or, optionally, <track>/<risk>. For example, ubuntu@22.04 or ubuntu@22.04/stable.