for RI model to work, the clean surplus must not be violated.
clean surplus is B(t)=B(t-1)+E(t)-D(t) where E(t)=(ROE-r)*B(t-1)
The idea is that if you have nonrecurring items on the IS that alter B(t) by an amount different that E(t)-D(t) then you CANNOT use the RI model. Said another way, for the RI model to be a valid, book value should only increase by E(t)-D(t).