der Sitz
headquarters
The main location or headquarters of a company, organization, or public authority.
Der Sitz des Unternehmens befindet sich in Frankfurt am Main.
The company's headquarters are located in Frankfurt am Main.

seat / headquarters
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headquarters
The main location or headquarters of a company, organization, or public authority.
Der Sitz des Unternehmens befindet sich in Frankfurt am Main.
The company's headquarters are located in Frankfurt am Main.

'Der Sitz' (seat/headquarters) is the noun related to the verb 'sitzen' (to sit). Many meanings are connected with the idea of sitting or of something or someone having a fixed place.
Nehmen Sie bitte Platz, hier ist noch ein freier Sitz.
Please take a seat; there is still a free seat here.
To describe the main location of a company, the fixed structure '[Firma] hat ihren Sitz in [Ort]' ([company] has its headquarters in [place]) is used.
Die Europäische Zentralbank hat ihren Sitz in Frankfurt am Main.
The European Central Bank has its headquarters in Frankfurt am Main.
In a political context, 'Sitz' (seat) is often combined with verbs such as 'gewinnen' (to win), 'verlieren' (to lose), 'verteidigen' (to defend), or 'erringen' (to gain) when referring to mandates in a parliament.
Die Oppositionspartei konnte bei der letzten Wahl zehn Sitze dazugewinnen.
The opposition party was able to gain ten seats in the last election.
To describe the fit of clothing, the fixed expression 'einen guten/schlechten/perfekten Sitz haben' (to have a good/bad/perfect fit) is used.
Dieses Sakko hat einen perfekten Sitz.
This jacket has a perfect fit.
When talking about posture while riding, people often speak of a 'fester' (firm) or 'sicherer Sitz' (secure seat) in the saddle.
Ein guter Reiter hat einen ruhigen und festen Sitz.
A good rider has a calm and firm seat.
place / space
'der Platz' and 'der Sitz' can both refer to something where you can sit. 'der Platz' often means a specific spot that is free or reserved for a person, for example in a movie theater, on a train, or at an event. 'der Sitz' refers more to the actual seat itself, for example the seat in a car or airplane. So you can reserve a 'Platz' and then sit on a 'Sitz'. There is a risk of confusion especially with means of transportation and events, because both can sound possible there.

Der Sitz im Bus war sehr eng, aber sauber.
The seat on the bus was very narrow, but clean.
Im Kino hat Anna einen Platz in der letzten Reihe reserviert.
At the movie theater, Anna reserved a seat in the last row.