ONLY E/Z system uses a priority system based on the highest atomic number. For assigning E/Z geometry, you compare atomic numbers of the two atoms attached to the same carbon on one side of the double bond. The atom with the higher atomic number is priority for that side. If the two priority atoms are on the same side (parallel with the double bond), it is assigned Z geometry. If the two priority atoms are on opposite sides, it is assigned E geometry. See attached picture #1.
Cis/trans is a different system completely. For an unsubstituted double bond, you base the relative geometry off identical substituents. If the double bond is in a carbon chain, the parent chain is what you follow. Please see picture #2.
Since cis/trans rules vary (it is confusing), I would highly recommend only using E/Z for specifying double bond geometry. It is very straightforward, and it will always work the same.
Cis/trans would be used for specifying relative geometries of a disubstituted cyclohexane for example. See picture #3.