Subjects allows you to tag chat with specific subject/topic. Defined subjects list can be found at
System configuration -> Live help configuration -> Subjects
Subject and Chat
In main chat interface on the right column you will see subject row.
+ you will see modal window where you can choose subject.
Afterwards you can search chats by subject in chat list window or you can generate chart by subject.
For operator to be able to set subject in the chat he has to have this permission
In order for operator to edit subjects he has to have this permission