The third factor is related to the configuration of the application. The application must have its configuration externalized, not shipped with the application itself. Shipping the configuration with the application requires altering the application bundle from environment to environment, breaking the integrity of the artifact.

To be completely agnostic from the environment, the configuration parameter must reside in the environment variables. In addition to environment variables, there are tools that provide better configuration distribution, such as Consul and Vault (security-related settings, such as passwords).

Long story short, have a separation of configuration from the code.