traefik/docs/content/routing/providers/service-by-label.md

In general when configuring a Traefik provider, a service assigned to one (or several) router(s) must be defined as well for the routing to be functional.

There are, however, exceptions when using label-based configurations:

1. If a label defines a router (e.g. through a router Rule) and a label defines a service (e.g. implicitly through a loadbalancer server port value), but the router does not specify any service, then that service is automatically assigned to the router.
2. If a label defines a router (e.g. through a router Rule) but no service is defined, then a service is automatically created and assigned to the router.

!!! info "" As one would expect, in either of these cases, if in addition a service is specified for the router, then that service is the one assigned, regardless of whether it actually is defined or whatever else other services are defined.