A source whose keys look like a properties file: database.max.pool.size.
database.max.pool.size
A source whose keys are the property names as written (KeyFormat.RAW), such as port. Keys of nested fields are the prefix plus the name: DATABASE_maxPoolSize.
port
DATABASE_maxPoolSize
A source whose keys look like environment variables: DATABASE_MAX_POOL_SIZE.
DATABASE_MAX_POOL_SIZE