Very good question and I am surprised that I as German needed to dig deeper into the stuff than anticipated. I tried to consult the German Wikipedia and the normal Google results, but got essentially nothing at first. I suppose it was self-evident schoolboy knowledge at their time and they did not bother to remark the function for following generations.
Finally I found it:
Reichstag 1903, Public Domain
Der große Sitzungssaal im Reichstagsgebäude (The big assembly hall of the Reichstag building).
- Tisch des Hauses (House table).
- Sitz des Reichskanzlers (Seat of Reich Chancellor)
- Sitz der Mitglieder des Bundesrates (Seats of the Bundesrat members)
- Sitz des Präsidenten (Seat of the President (of the Reichstag!))
- Sitz der Schriftführer (Seat of the Secretaries)
- Rednertribüne (Speakers platform)
- Sitz des amtlichen Stenographen (Seat of the stenographer)
- Journalistentribüne (Journalist lounge)
- Nein-Thüre ("No" door, "Thüre" is old German for "Tür").
It was the seats of the Bundesrat (1871-1918) / Reichsrat (1919-1933, just name change) which were members of the states of the Deutsches Reich.
More about the responsibilites of the Reichsrat here. While the Reichsrat itself resided in another building, its emissaries were present in the Reichstag on both sides of the central stage.
The seating order of the parties generally was mirroring their classification of left and right. At the most left the communists/SPD, at the most right the Centrum (conservatives)/NSDAP.
Seating order in Baden-Württemberg (not Reichstag!), Public Domain
Just to explain the description 9: The Reichstag had a special voting scheme where members of the Reichstag entered the hall through two doors, the "Yes" / "No" door. The procedure was called Hammelsprung (wether’s leap) and the architect Paul Wallot depicted Polyphemus stroking the sheeps back for "Yes" and the old German mountain fairie Rübezahl for "No".
Interestingly the seating order has not much changed for the current Bundestag. The only difference is that the state representatives are exclusively sitting on the left together with the Wehrbeauftragten, the Parliamentary Commissioner for the Armed Forces.