What is a sessionhash ?

[8utt]

Whats the difference between

SESSION_IDHASH
and
SESSION_HASH

The session hash is created uniquely once every visit ?
The session Idhash is created uniquely for the same computer ?

So if I visit the same site from the same computer i get a new session hash but keep the same session idhash ?

If someone could put me right or confirm that thats how its supposed to work that would be great.