12 9s is new to me too. Not worked on anything better than 5 9s (5 mins downtime/year), and Wikipedia only goes to 9. 9 9s comes to 35ms downtime a year. I can't think of anything that needs more than 5 9s, let alone 9.
This is a durability metric, not an availability one. This typically tells you the likelihood of losing a given object in a year. (5 9's would be quite poor for this, implying a loss of 1 object out of 100,000 every year)
In this case, it's not as much service uptime as it is data retention. If you're storing 5+ PB of data, even 5 9s of data loss per year can have a measurable impact.
http://en.wikipedia.org/wiki/High_availability#Percentage_ca...