Service mapping is a way for you to tell ShipStation what shipping services to automatically assign to an order based on your customer's Requested Service during their checkout.
For example, your store might offer Free Standard Shipping, 2-3 day Shipping, and Overnight Shipping. Once that order imports into ShipStation, you can configure your Service Mappings to automatically assign the actual shipping service you want to use, based on what your customer chose.
Select an order in your Orders grid, make sure the orders Sidebar is showing, and click the link that says "Unmapped" (as shown here):
The "Requested" value in orange is the Requested Service selected on your store by your customer.
In the Service Mapping popup, select the service you'd like for that requested service:
Save the changes. The "Unmapped" link will now say "Mapped."
Going forward, all orders importing from that selling channel with that Requested Service, will automatically have UPS Standard assigned to it. Automating your workflow like this is just one way to making your process as efficient as possible!
If you'd ever like to change the mapping to another service, just click the "Mapped" link in the shipping sidebar and edit the service. Alternately, you can go to Settings > Selling Channel > Store Setup and click Edit next to the selling channel you'd like to edit mapping for, then go to the Shipping Services tab. All your service mapping for that selling channel will be listed here for you to view and edit any time you need.
NOTE: all service mapping is store specific, so if you have multiple selling channels connected, you will need to map services for each store you want to use service mapping with.