use e/z when you have a POLYsubstitued compound. so the alkene has more than just Cl and H as its substituents it has like a butyl and a bromo and H and propyl grp then its e/z.
Cis and Trans is typically when the substituents on C1 and C2 across the double bond are the same, E and Z are cis like and trans like meaning that the substituents are not the same and the higher atomic number will determine the conformation.