This key is mandatory for the so called
local functions, i.e. those functions that are associated with some existing Deriscope Object.
In those cases, it specifies the
handle name of the associated object.
A typical example is the
Deriscope Function Price, 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.