The deployment configuration file has a ClientCode and Environment node.
At 15below we use Octopus Deploy which means the Environment is populated by the deployment environment rather than the deployment configuration file.
We also set a ClientCode value as part of the Octopus project and name the files [ClientCode]-[Environment]-Config.xml.
But if you are not using Octopus, the ClientCode and Environment nodes can be useful for setting up these default values.
<Structure xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<ClientCode>AAA</ClientCode>
<Environment>PRD</Environment>
...
</Structure>