XXX: Scrap entirely? This is not really relevant
If you thought that was the end of the story, and you could put the Unix Hater's Handbook back on the shelf, here comes another sort of uid. This one is a Linux specific extension, the fsuid, or file system uid, which is the uid used in all file system operation. Normally, it shadows the effective uid, i.e. if you change the effective uid, the fsuid is changed as well. However, an application can explicitly manipulate the fsuid and fsgid using the setfsuid and setfsgid system calls, respectively.
The main reason for introducing the fsuid was the case of network daemons that want to temporarily ``become'' a certain user when opening a file, etc. If the daemon changes its effective uid, it also allows that user to send it arbitrary signals, allowing the user to kill this daemon. Normally, this is not a huge problem. Sendmail, for instance, sets its effective uid when opening a user's .forward file. If a user sends this sendmail process a KILL signal all he achieves is disrupt his own mail delivery; the main sendmail daemon continues to run. In fact, I know only one network service that has ever really used the setfsuid system call, the Linux user space NFS daemon (and to some extent, setfsuid was created because of it).
Most of the time, you won't need this feature, but it's good to know it's available in case you need it.