According to the official reference guide, YAML is the recommended way to configure an application in Wildfly Swarm, and the user-provided YAML file is
applied overtop the absolute defaults that WildFly Swarm provides
What are these absolute defaults? The documentation does not say anything about that.
EDIT:
A few defaults are shown in https://wildfly-swarm.gitbooks.io/wildfly-swarm-users-guide/configuration_properties.html, but the defaults for most fractions are missing: Logging, Batch, Mail, Ee, Ejb, Io, Remoting, Transactions, Webservices etc.