I quote again my interpretation on the point:
"Exceptional" is - both in Italian and in English - quite an unspecific word itself. It means that it is not the ordinary thing.
As for what I have read in the SC rulings, the legal accepted meaning for "exceptional" is that it must be something with a quid pluris compared to the simple ground for cautionary custody. That is, the same (usually unspecific) grounds for cautionary custody are not sufficient to apply § 3 and 4 art. 104. There must be some additional more specific reason. It must be "exceptional" in the sense that it is additional to the ordinary reasons for which cautionary custody applies, and it must be a reasons of the moment, related to the immediate needs to not jeopardize the investigation (it canot be a protracted reason).
Now, you may have a look at this book:
In google books
Just this pagereports and explains the SC jurisprudence on art. 104; it repeats all the preliminary conditions which LJ pointed out and quoted, says actually even more, explaining how jurisprudence points out what
cannot be considered “exceptional” as a ground to apply that caution.
In the end the questions comes down to a “quid pluris” – a “something more” that must belong to the situation which makes the ground exceptional and specific enough for the application of par. 3 and 4., in addition to the simple arrest or cautionary custody.
And jurisprudence, in fact, enlists
three specific “dangers” (“grounds”) which would make the reasons for caution “exceptional” enough for the application. So what is the “quid pluris”? This can defined by three situations (I quote from the text on google books):
Si richiede pertanto un quid pluris rispetto ai requisiti cautelari al fine di scongiurare
a) La precostituzione di una strategia comune a più coindagati
b) Tesi difensive di comodo
c) L’utilizzo di un linguaggio criptico fra più coindagati per comunicare tra loro
translation:
The “quid pluris” with respect to simple grounds for cautionary measure is required when there is a need to prevent (a danger of):
a) pre-constitution of a common defensive strategy between multiple co-suspected
b) setting of opportunistic defensive theories
c) the use of a cryptic language between co-suspects to communicate between them
A personal comment: about the cryptic language. As I read Amanda’s hand written memoir for the first time I jumped on the chair when I read about her “memory” of “bloods on Raffaele’s hands” that she says remembers, but thinks is tas “probably” “fish blood”. I felt chilled because this is exactly the kind of cryptic language which I associate to the most dangerous criminals (those who commit crimes in association) and I interpret it as pure mafia-style communication.