@StaffanH Do you have a question?

Yes. What does it mean here,  supplement required?

It means as an interrail pass holder you have to pay a supplement on some service that cross borders, for that journey maybe see if you can get a Berlin to Poznan train and then Poznan to Krakow, also look at the polish apps (PKP intercity) as well as the German app (DB Navigator) for cheaper reservation options.

DB navigator is offering a seat only reservation on the 10.52 for €5.20.

hope that helps