[Koha] Hidden attribute of subfields in cataloguing frameworks

[Stefano Bargioni]

In my Koha 3.2.0 test installation, hidden attributes of subfields have these values:

9
7
6
5
4
1
0
-1
-5
-6

I'm not able to find the meaning of each code in manuals, FAQs, forums...
Thanks. Stefano