etcupdate usage

Hello,

I do not understand how to use etcupdate. I am lost in the different options. 
s there somewhere a tutorial on its usage ?

PS: on a machine that juste went from 14.2 to 14.3, etcuodate diff shows diff, but etcupdate says 
Failed to build new tree.

Is it because src is not on the machine ? (not enough disk)

I soon will upgrade another machine (with /usr/src this one), and I'd prefer to be sure to be able to use etcupdate on it 
(especially if I need to to a extract or build before upgrade)

-- 
Erwan David