Hi Stephane,
Categories are used to group content together, not used to apply security.
Any security should be placed on the folders where the reports are located.
The issue you are facing is probably that Everyone has access to all folders comes from the setting on the root folder (top level folder) which gives Everyone Full Access, which is very dangerous if that's the case. Remove that!
So in your example I would create the following setup (simplified):
Folder "C1"
contains report doc0
Deny access to Everyone
Give Full control to group "BI Mobile C1"
Folder "C2"
contains reports doc1 and doc2
Deny access to Everyone
Give Full control to group "BI Mobile C2"
If you do that, the security should be correct and should be working as expected.
Regards,
Josh