Yes there is seating, but it often fills up. My advice is to get there a little early. If you can't do that, waiting a little while will often get you a seat when someone leaves.
Most of the time there is. It's better to get there early so you can actually get seating. Every time I've ever been to a concert hall of opera hall concert, there has been a production of some sort on the Millenial stage and seating provided.
I've been to a performance there with seating, and have seen the stages configured with seating when I was at the Kennedy Center for other performances. There's also informal seating on the stairs adjacent to the stages, which can be particularly good for young kids.
I've been when there was seating for everybody and just standing room only. I think it depends on the concert.
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No, it's just standing room in the hallway. But if you have a disability and are unable to stand you should contact them in advance as I have seen them set up seating for people who need it in the past.
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