Key Apply Fallback of function in refers to a wizard-only boolean flag that functions as follows:
Web blog examples here and here
When TRUE, the default collection of objects generated by the wizard for the key will include an object of type that contains the specifications needed for dealing with indices after their cessation time.
The existence of that object in the input model collection will affect the pricing outcome to the extent the latter depends on fixings of indices at future times after their scheduled cessation.
When FALSE, no such model will be included and the ensuing pricing outcome will assume that all indices remain valid for ever.