Available with Network Analyst license.
Your network analysis layer supports using hierarchy but that setting is turned off. If this layer is published to a map service, it may not perform as well as expected.
Take any one of the following actions to resolve this issue:
Do nothing. Mark this warning as an exception to avoid seeing it again.
Automatically correct this issue by choosing the appropriate option on the warning's context menu.
Open the layer properties of the service area layer, click the Analysis Settings tab, then check Use Hierarchy.
Hierarchy makes it possible to compute long, nationwide routes quickly. Generally, you should enable hierarchy in published services.