Key Notional 1 in refers to the optional capital amount upon which the rate during the first accrual period of the first leg acts in order to accrue the respective coupon amount for that first period.
If omitted, it equals 1.
On the absence of amortization, this same capital amount also applies to the subsequent accrual periods of the first leg.
When amortization is present, the applicable notional amounts are shown at
Look at for how the notional affects the payoff in formal terms.