There are two places balena.yml can be according to the documentation. One place is utilised for secrets in .balena/balena.yml of your project.
The second place balena.yml can be according to the docs is in the root of your repo, where it is used to define things for your balena deployment such as environment variables or host variables.
What is not stated is the significance of CONFIG_MODE, or how to state service variables rather than app variables. Perhaps there are also other undocumented features. Thanks 