You can add up to three pairs of additional information to each of your active customers from within your GoCardless dashboard. These are custom fields and can be used for things such as custom references, account numbers, or any other other additional information that will help you identify and filter your customers.
To add a reference to a customer, first click on the customer via your customers page, and then click the edit button in the top right.
In the window that opens, you will see a section below the customer's email address called Custom fields. 
Tick the box to the left and the section will expand so you can add your reference.
You will see that references are formed of two parts, Name and Value. The name is the label you would give for the type of reference you are adding and the value is the actual reference you are giving the customer.
e.g. Name: Membership_Type, Value: Gold Package.
Once you’ve added your reference click Update customer
You will then see the reference on that customers page at the bottom of their ‘details’ section.
You will also be able to view these custom references in your customer overview page.
As you can enter up to three pairs of custom references for each customer, you can also filter between these on the Customers page. To do this, click on the pencil icon to the right of the custom field title at the top and a dropdown menu will open, allowing you to select between them.