Shipping rules are a powerful tool within Shiptheory and allow you more control over the data you send to carriers. One of the functionalities within shipping rules is to override certain values. For example, we can set an override rule which will populate the delivery instructions field of a shipment with a custom message every time. 

With the "Matches" operand we can now force the value of one field to equal the value of another. We'll use what3words and delivery instructions as our example: 

Firstly we need to navigate to our shipping rules, select a rule and scroll down to the 'Update Shipment Details' section of the rule's advanced settings.

Then we need to select a value from the dropdown menu. We'll choose 'Delivery Instructions'

After that select the option of 'Matches' in the central dropdown select. 

Then we'll select the option 'what3words' from the final dropdown select on the right. 

Finally, we'll hit the green plus button to add our override rule. Remember to click save before leaving the page otherwise your changes will be lost.

Now every time a shipment comes into Shiptheory which matches the conditions of the rule the value of the shipment's what3words address will be pasted into the delivery instructions (note: anything in the delivery instructions field will be overwritten by this operation).