When you use category or key value targeting on line items or creatives, it can be hard to remember how a categories (as defined by the URL structure) are designed on your site, or which key values are being sent with the ad requests. Reference data lets you upload a text file that creates a library of categories and key values that you can choose from, rather than remember them all.
Category uploader: Just create a .txt file where each line specifies a new category that you want to add. Below is a template that you can download and use if you are in doubt.
Key value uploader: Just create a .txt file where each line specifies a new key-value entry and commas separate the data like so: mykey, myfirstvalue, mysecondvalue. Below is a template that you can download and use if you are in doubt.