Protons interaction

Dear all,
I’m new Geant4 user, so i simulate protons beam of 230MeV on heterogeneous multilayer phantom and i was asked question that i didn’t understand… so can some one help me on it???
the question were :You have to determined the effect of the the size of the target to introduce uncertainty in the determination of whether a recoil proton of a particular energy stops in the target.
Thank you in advance.