The “quota” applies only at an org of 2k+ (or may be 1k+, I need to re-read it). There was a similar system in the past without any guidelines. Historically that was 2-5%. Google s rolling out a new system and they are saying the bottom 2 ratings are expected to be about 6%. Given what I saw over the past many many years at Google, it won’t be an issue. But the company sucks when it comes to communicating this to the employees and causes stress to low level managers until they go through one or two cycles.