Juju 1.18.x is the first Juju that is aware of two Ubuntu LTS releases. In the past, when bootstrapping the state-server or deploying a charm, Juju selected Ubuntu 12.04 Precise when the serie was not specified. If you care about the version of Ubuntu used, then you want to specify the series

You can specify the series of the state-server and set the default charm series by adding "default-series" to environments.yaml. Setting "default-series" is the only way to specify the series of the state-server (bootstrap node). For example, if your local machine is Ubuntu 14.04 trusty, but your organisation only supports Ubuntu 12.04 Precise in the cloud, you can add this to your environment in environments.yaml:

default-series: precise

There are many ways to specify the series to deploy with a charm. In most cases you don't need to. When you don't specify a series, Juju checks the environment's "default-series", and if that isn't set, Juju asks the charm store to select the best series to deploy with the charm. The series of the state-server does not restrict the series of the charm. You can use the best series for a charm when deploying a service.

When working with local charms, Juju cannot fall back to the charm store, it falls back to the environment's "default-series". You must specify the series in the environment or when deploying the charm. If your environment is running, you can add "default-series" like so:

juju set-environment default-series=precise

These commands choose Ubuntu 12.04 Precise when "default-series" is set to "precise" in the environment:

 juju deploy cs:precise/mysql
 juju deploy precise/mysql
 juju deploy mysql
 juju deploy local:precise/mysql
 juju deploy loca:mysql