Testing terraspace I found that config directory seems to has layer properties. I can put config directory and its configuration tree inside stacks and all configurations inside stacks
app/stacks/stackname/config/
have precedence over configuration under
config/
This is great as it allow to customize many configurations at stack level (providers, locals, backend, …).
I can’t find this configuration layering in the documentation. It is by design ? I lost some piece of documentation ?
Think you’re looking for https://terraspace.cloud/docs/config/
It’s also configurable and overridable within each module itself
If stack config exists, it’ll take higher precedence over the common config folder. Docs could be improved here. 
Terraspace follows some Ruby philosophy: things should generally behave how you expect them to behave.
I read this document page, it was not so clear that config could be defined in module and stack also. This make configuration more flexible. A point in favor of terraspace.
Thak you.
One more great improvement could be locals based on environment (as done with tfvars), so using in locals different data structure and in main same flatten function give more flexibility and cost control between ENVs.