This settings allows you to set a Default Unassigned Rate.
Hub Planner looks your rates with the following hierarchy:
If Unassigned Work has a custom rate, we will use that for that Unassigned contribution to the project. This is useful if certain Unassigned Work Categories are billed differently than what the project rate is. e.g. Senior Staff
If there is no Unassigned Billing Rate, then we will use the Project Rate which could either be custom or default
If there is no project rate set, we will use the company rate