jail parameters

[rick]

I'm trying to establish a jail on a FBSD 6.1 system and have a question
about a couple of the parameters.
Under the jail man page there are two  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
rick