It is up to the creator of Heat templates to decide what resources are considered as baseline already existing in the cloud, and what resources will be created by the Heat template.
Automation examples for Openstack using the HEAT templating language
HEAT is Openstack's template-driven orchestration engine. It allows users to define servers, networks, storage etc as "YAML" (https://en.wikipedia.org/wiki/YAML) templates which can then be deployed as "Stacks" to create complex systems which are scalable, repeatable and automatable. See: https://wiki.openstack.org/wiki/Heat for more info on what HEAT is.
- Heat template guide - Heat CLI reference - Ansible module for using Heat