As a best practice, Cortex recommends anything that is code-first be defined as a service, while anything non-code should be defined a resource.
For in-house libraries that are code-first entities with metadata — such as an owner, repository, or best practices — we recommend representing them as services in the catalog.
For external libraries — such as a dependency on an open-source library — we recommend representing them as a package dependency in the catalog. In this case, they would not be defined as a service or resource. (You can read more in our dependency documentation.)