According to this model, a planetary body is assumed to be composed primarily of silicate minerals and NiFe (i.e. a mixture of nickel and iron).
According to the rain-out model, core formation was a relatively rapid process, taking a few dozen millennia to reach completion.
Furthermore, it was only after all of the iron and nickel delivered by impacting bodies had arrived that core formation could proceed to completion.
However, this process of core formation was preceded by a long period of partial differentiation, in which some of the nickel and iron within the planetary embryos had begun to separate.
The rain-out model can be invoked to explain core formation in all the terrestrial planets, given that these consist primarily of silicates, nickel and iron.