Key Issuer in
Yield Curve refers to an optional object of type Issuer that identifies the risk profile of the implied discount factors.

Web blog example
If omitted, the
Riskless Issuer is assumed by default.
This entry, together with the
Currency, identify the Quotable part of the Valuation nature of the corresponding Yield Curve object.
All other entries - eg conventions and market rates - represent the remaining
Quotable Value part.

In practical terms, all Yield Curve objects in a pricing scenario must have unique pairs of Issuer and Currency
The pricing algorithm will ask - as an example - for a Yield Curve with a specific currency - eg USD - and a specific issuer - eg
Riskless Issuer -.
If there exist more than one Yield Curve objects with the requested currency and issuer, the pricing will fail due to ambiguity as to which curve should be used.