Hi,
When CI based routing is enabled for a category, you specify the contact from the CI to be used as the Group for the ticket. The CI will then have to have a Group on the contact field that you chose for the CI based routing.
When the selected category has CI based routing enabled, the CI will determine the Group on the ticket (only when Group is not set during creation).
Example:
CI
Area
Results (notice Group is not set)
When Save is pressed the Group is then set to the Support Contact 1 from the CI.