This varies a lot. As a typical loop starts with a pre-brief where the bar raiser, hiring manager, and other participants should outline the key skills require to succeed in this role and then they divide up the skills / competencies that each person will assess.

So in that regard the bar raiser will cover different topics in different interview sessions. That said, the biggest thing about bar raisers is they've interviewed a lot of people and received specific training around what to press on and how to tease out the strengths and weaknesses of a candidate. As a result, the good bar raisers are really good at asking the follow up question, as well as, asking questions in ways that force you to expose your natural tendencies.

Also in most of my interview loops there the bar raiser typically asked behavioral questions. So I would recommend prepping on your background and being able to speak to why that prepared you to succeed in this role.