Notional

Key Notional in Fwd Contract refers to an optional number of units of the underlying tradable defined in Underlying
If omitted, the default is 1.
Note that the underlying tradable may itself define its own notional.
For example, if the Underlying is a USD bond with a notional of 1,000,000 and the notional here is set to 2, the forward contract references effectively a USD bond of a notional of 2,000,000.

It follows that in the special case of FX Forward, the number here equals the number of units of the currency Tgt Ccy
For example, consider an FX Forward with Src Ccy = USD, Tgt Ccy = JPY and Strike = 120
If the contract's term sheet specifies a notional of 1M USD, it would be a mistake to enter 1,000,000 here, as this would be interpreted that a 1M JPY (the target currency) would be exchanged at maturity.
The correct notional entry would be the number 1,000,000 * 120 = 120M since this number of JPY units would imply (according to the strike of 120) that 1M USD would be exchanged at maturity, in conformance with the term sheet.