Velocity

i should stop reading on the subject, the more i read the less i know. some contradictory info is now making me doubt in my basic logic capacities :P

is anything false here?

1- velocity is used by dev team to estimate how much work they can add in a Sprint backlog
2- the only purpose of velocity is to help p.o. make forecast (!!!)
3- velocity is updated by anyone that sees it needs to be updated
4- velocity of a spi can only be accounted for when the spi is done (which is the same as 'when the po considers it done'

(i find #2 to be a little extreme, by saying that its the "only" purpose.. but i can see how it can be useful to the po)

thanks!