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This type is exclusively used to describe the volatility of forward interest rate swap rates.
It thus only makes sense if the entry
Key Vol Spec::Ref Quotable defined within Vol Spec relates to a Swap Rate.
The fundamental assumption is that for a fixed expiry T1 and underlying swap maturity T2, the forward rate F(T1,T2) is diffused in a
Vol Spec::Vol Type::Black, Vol Spec::Vol Type::Normal or Vol Spec::Vol Type::Shifted Lognormal fashion.

Note that we do not assume that the same diffusion parameters apply to all different combinations of T1 and T2.
In other words, for each pair T1, T2, we allow the respective forward rate F(T1,T2) to diffuse with a different vol parameter σ that depends on T1, T2.

This gives rise to a non-flat volatility surface σ(T1,T2).
For a given discrete collection of pairs T1, T2, the surface becomes a 2-dimensional grid.

Deriscope allows you to specify this grid of market vols as a
Table2D object containing volatilities for various T1, T2 combinations.
One dimension must span the option expiries (entered as dates or steps) and the second the swap tenors (entered as steps).
Bilinear interpolation is assumed for any missing entries in the supplied table.
The extrapolation type is controlled by a separate input.
The QuantLib implementation is the SwaptionVolatilityMatrix.