The Customer custom fields or just custom fields is a functionality in the Customer section that allows you to collect more data from your clients. For example, You can request gender, age or other data that maybe is required information for your business.
How do I add a custom field?
You can check the example below:
This is how it would appear in your store's checkout:
Subsequently, the data collected can be used to segment audiences by different metrics. For example, you can make a targeting campaign for all female users between the ages of 20 and 45.