Ok. So I am taking ICE Hamburg - Berlin and ICE/RE Berlin/Kempten in 1st class with a seat reservation

Since I am from India I guess I will use the analogy from there. While travelling in India if you hold a particular class ticket you are assigned to a certain coach on the train and a certain seat/ berth.

So on the ice trains- are you also assigned to a certain coach/ seat or its where ever you can find an open seat.

Thanks for clarifying.

Take any seat you want,even if you have a seat reservation you are not obliged to take this particular seat.

Not quite. You can take every non-reserved seat you want. Above the seats on the wall or on the overhead compartment you will see small displays. The display will show the name of the stops where the seat is reserved. In your case it should say Hamburg-Berlin. If nothing is shown on the display the seat is not reserved.

Best is if you have a seat reservation to stick with your reserved seat. The ticket will tell you the coach number and seat number.

If you have a reserved seat, stay with it. I learned my lesson the hard way on a southbound ICE from Frankfurt, one miserable Friday evening:
1. Got on the train. Second class ticket + reservation.
2. Found my seat. Big sweaty bloke in the seat next to mine.
3. Hmmm... these seats over there look kinda empty. Are they reserved?
4. No reservation on the display - it read "ggf freigeben". So I take a very nice window seat in a group of four around a big table in the middle.
5. My original seat is promptly taken by somebody else. I don't mind at all.
6. Next stop some time later (Mannheim), and a family walk in holding reservations for the seat I'm in (and other seats nearby). I have to give up my seat.
7. The guy in my original seat refuses to move. I call the conductor, to no avail, because (a) "ggf freigeben" means that these seats are subject to last-minute reservations and (b) a reservation is void if you do not occupy your assigned seat within a certain time after the train has left the station where you boarded the train (15 or 20 minutes). In my case, that time had passed. So the guy in my original seat was perfectly within his rights and stayed put.
8. The train was now very much full. As in, no free seats and not much standing room left either. So I travelled standing up from near Karlsruhe to Freiburg, where a lot of people got off.

Tom

vielen dank!