Flat Rate

Key Flat Rate in Yield Curve refers to the flat zero rater associated with this curve.
Enter .04 for a 4 % rate.
Here the rate r assumes the daycount and compounding conventions defined in the entries TS Daycount and Compounding respectively.

Concretely, the curve is built in such a way that it implies the same zero rate r for any maturity, provided the daycount and compounding conventions are as above.

Note that setting a flat zero rate here does not mean that the implied zero rate with respect to differing daycount or compounding conventions is also flat!

Also note that the implied forward rate - even with respect to the same daycount or compounding conventions - will generally not be flat either.

An exception is when the compounding convention is set to continuous, in which case the continuous forward rate will also be flat.

Note, the Wizard creates by default a Yield Curve by setting the Flat Rate to some arbitrary value as this corresponds to the simplest possible setup.
For real applications though, it is recommended to build the Yield Curve using the optional key Market Data and supply as input actual market rates of various types of traded instruments.