It's considered an uncontained failure if debris leaves the engine through a hole that didn't previously exist - if it falls out the back (or manages to bounce

It's considered an uncontained failure if debris leaves the engine through a hole that didn't previously exist - if it falls out the back (or manages to bounce off the containment structure and exit the front), that still classifies as contained. Obviously the difference to people on the ground is pretty semantic[1], but the assumption is that a contained failure should have very little chance of putting the aircraft at risk. [1] Well, as long as the failure doesn't result in an entire plane landing on their neighbourhood instead of just a few parts