Custom service codes are a hidden string of text that travel along with a shipping rate after a shipping rate has been added to an order to be read by 3rd party systems.
You can think of a service code as a 'handle' for a shipping rate. A service code let's you change the title of a rate as often as you want without confusing a 3rd party system, because the hidden service code will remain the same, if you set one.
If you don't know what a service code is, then you don't need to worry about it!
However, if you do want to be able to specify custom service codes for the rates from the app you can do so by activating the Custom Service Code feature in the app. This feature requires the Pro or Unlimited plan.
How to activate Custom Service Codes
Go to the Settings tab in the app
Scroll down and find the section Custom Service Codes
Click 'Enable Custom Service Codes'
You may be prompted to upgrade your plan, if so, follow the steps to do so
Once you activate it, new fields will appear within every rate to set a custom service code.
How to use Custom Service Codes
Once you activate the Custom Service Code feature, a new input field will appear in the title section of every rate in the app:
Just edit the Service Code field to be whatever you want it to be.