This key is mandatory for the so called , i.e. those functions that are associated with some existing .
In those cases, it specifies the of the associated object.
A typical example is the , which is local since it makes sense only in association with a specific object.
For example, consider two separate objects, an interest rate swap with the handle name "Obj1" and a bond with the handle name "Obj2".
Then providing only the function "Price" is ambiguous, because one cannot know if it refers to the price of "Obj1" or "Obj2".
The ambiguity is resolved by supplying the handle name of the associated object, eg "Obj2", through the key here.