when are you officially all in?

[mostsmooth]

suppose you are playing say 100-200 limit
you happen to have 100 left on the flop and you bet it.
are you now all-in, or do you only become all-in when theres a new bet/raise you cant cover?
the only reason i ask is that if you become allin during a hand, if you want to buy more chips after the hand (assuming you lost), you have to buy at least the table minimum, if you werent all in, you could buy less than the minimum.(at least thats the rule anyplace ive played)