This directory is intended to contain drush configuration that is not site or environment specific.
Site specific drush configuration lives in sites/[site-name].
You should add the Drush aliases for any remote environments (such as Acquia Cloud) to the site-aliases directory. This allows developers to access remote environments using simple aliases such as drush @mysite.dev uli. Note that if you are using Acquia Cloud, developers can also download these aliases manually from their Insight account, but providing them with the project makes everyone’s life a little easier.
You can find these aliases for Acquia Cloud sites by logging into https://insight.acquia.com and going to the Credentials tab on your user profile. Download and place the relevant alias file into site-aliases.
Note that if the version of Drush that your project uses is significantly ahead of the version available in the remote environment, you’ll need to manually set $drush_major_version at the top of your alias files to match the version of Drush on the remote environment. For instance, at the time this document is being written, Drush 9 is in development and only Drush 8 is available on Acquia Cloud, so you’d want to add the following to the top of your aliases: $drush_major_version = 8;
It can be helpful to define aliases for a local environment such as @mysite.local. This creates consistency with how aliases are already defined for remote environments (such as @mysite.dev, @mysite.test, and @mysite.prod). To create these local aliases, copy example.local.aliases.drushrc.php to local.aliases.drushrc.php and modify the default values as appropriate for your project.