We assume a latent variable R (pRoperties of entities in the world), and build a generative model (that is, model the joint P (I, R, U )).
going from P (I, R|U ) and making certain independence assumptions, we arrive at we assume that R is only conditional on I, and U is only conditional on R, and we can move P (I) and P (U ) out of the summation, as they do not depend on R. This is an update model in the usual sense that the posterior (P (I|U )) at one step becomes the prior (P (I)) at the next.