The "seat" of arbitration is the legal home of the proceedings — it fixes which country's arbitration law applies, which courts have supervisory jurisdiction, and where an award is treated as having been made, regardless of where hearings actually happen.
The "venue" is simply the physical or virtual location where hearings take place. A tribunal can sit in Chennai for convenience while the seat remains Mumbai, London or Singapore — the seat does not move just because the parties meet somewhere else.
Contracts that use "seat" and "venue" interchangeably, or name only a city without saying which one they mean, invite an early fight over jurisdiction before anyone reaches the merits. State both terms expressly, and make sure they say what you actually intend.