UNIX for Advanced & Expert Users

I'm trying to establish a jail on a FBSD 6.1 system and have a couple of questions on bringing up the daemon.
Under the jail man page there are two user flags that I am unclear on,

-u username The user name from host environment as whom the command
should run.

-U username The user name from jailed environment as whom the command
should run.

I think that I understand what the -U is for, but wouldn't bet the family silverware on it, as for the -u flag I am completely lost.
Could someone give me a clue on these two items?

Thanks
thumper

I suppose the "-u" implies the real user name on the system, vhost, or something else, where the "-U" is the jailed one, i.e. you can have different credentials.