Although this may not be helpful, I am not familiar with that threshold within any of the fair lending related materials. I would assume that this must be some other guidance unrelated to fair lending. I know that FNMA & FHLMC have 15% within some of the MBS language, but I don't know if the OCC issued anything regarding a 15% threshold as part of any repurchase risk guidance.
The fair lending information they have published is pretty generic. And the examination procedures only refer to sampling of the groups to see if there are any inconsistencies. So I don't think it would be part of any fair lending guidance.
As I recall, our examiner suggested a threshold of 5% for exceptions and I have worked off of that as an indicator of "low" or "managed" risk for assessment and monitoring purposes.
Again, probably not helpful, but I thought I would respond.