COUNT DE MONET
10808
There is no uniform definition at all for a grand complications
It is, as Nicolas is saying, a marketing tool to call a watch a "grand complications".
To give you an answer to your question: the movement of the 5959 is much, much more complicate to produce than the latest chrono split second.
That Patek has not used it for any other watch is showing that it is a purpose built movement where the sole purpose was to be slim!
I have to look / read again in literature to tell you more about the differences in detail but what I can recall is that the movement is made from German, assembled twice and is the slimmest split second chrono to this day.
By the way: a slim, reliable and accurate running movement, regardless any other function is still the biggest challenge in traditional watchmaking.
The new split second movement has got a friction reducing isolator mechanism that the 5959's is lacking.
As I mentioned there are more details but I can not have not available at the moment to tell you.
Moritz
The new 5370P is not accounted for as a
It is a marketting thing. A split seconds cannot be, if not accompanied by...
According Patek
There is no uniform definition at all for a grand complications
Thus it a complicated complication
Indeed, and I would say it is well deserved
The 5959's CHR 27-525 PS caliber was indeed the slimmest Split-scond Chronograph movement
About the isolator
There is an isolator