Maturity
The column titled #Maturity in the table associated with the key Vol Setis mandatory and contains elements, all of which must be of type either Date or Step.
Each element represents the maturity time associated with the corresponding volatility quote.
In formal terms, the underlying assumption is that the underlying quantity Q (stock price, fx rate or something else) starts at time t = 0 with some known initial value Q₀ (referred to as spot price) and thereafter is diffused according to a lognormal process
The maturity here essentially defines the time t, by using the assumption that t = 0 corresponds to the trade date