Popularity based recommendations. What is "Profile Attribute to Match" ?

I want to create a "most popular in your favourite brand" recommendation, which with the new update it should be through the "Popularity-based" recs. 

What does the field "Profile Attribute to Match" do on the new "Popularity based" recommendations? 

Not sure why Adobe released an update without also making a proper documentation at the same time. 

Currently the documentation says "(Info coming soon)" for Top Sellers by Item Attribute and Top Sellers by Item Attribute and Most Viewed by Item Attribute.  

https://experienceleague.adobe.com/docs/target/using/recommendations/criteria/base-the-recommendation-on-a-recommendation-key.html?lang=en