In string theory, we know three dualities.

S-duality  : Extension of Electric-magnetic duality, Duality between strongly coupled qft and weakly coupled qft. (One of the typical example is Seiberg duality in sqft)

T-duality  : In short $R\leftrightarrow \frac{1}{R}$

U-duality : In M theory perspective, U-duality usually means T+S duality.(???)

But I am wondering about the last part, U-duality.

What is U-duality and why it is important in string theory?

In string theory, we know three dualities.

S-duality: Extension of Electric-magnetic duality, Duality between strongly coupled qft and weakly coupled qft. (One of the typical example is Seiberg duality in sqft)

T-duality: In short $R\leftrightarrow \frac{1}{R}$

U-duality: In M theory perspective, U-duality usually means T+S duality.(???)

But I am wondering about the last part, U-duality.

What is U-duality and why it is important in string theory?