This top-level directory is for config specific to an AWS Account. It's called directly from ../main.tf.
Defaults should be used when possible: variables must only be specified when overriding a module's defaults is required.
As dev doesn't (yet) deviate from the defaults (it is a sandbox account: the defaults should therefore be configured for this account), it is a soft link to default. common exists for consistency with ../environment/common, which should be renamed in future to ../environment/default. At that point, the [common] symlink here should be removed.