[SQU] access.log questions

It appears from the access.log file that Squid caches some sites but not
others. It even appears that for a particular web site (i.e.
www.aol.com), only part of the site might get cached.
How does Squid determine what to cache and what not to cache? (no extra
acl entries have been defined beyond the defaults)
How does Squid know when to refresh a site that it has cached? How does
it keep the data up to date, especially if that site that is constantly
changing?

Also, I noticed that IP addresses are recorded sometimes rather than the
actual user id. I even see the IP address AND the user id of the same
user listed in the same access.log file.
Why does Squid log IP address instead of the user id sometimes?

Thanks,
Dave Burris

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