In the CPU priority list, what is << Background>> compared to <<Critictical>>?

In the CPU priority list, what is << Background>> compared to <>?

Is it best practice to set the “High Cost Reflections” and “Low cost Reflections” at this level of priority?

We have reflections which are taking a long time to get refreshed. Below our current settings:

@BBE The job profile will tell us why the refresh of the reflection is taking time. Can you please attach it for review?