In 1-bromo-2-chloroethene, $\mathrm{Br}-\mathrm{CH}=\mathrm{CH}-\mathrm{Cl}$, the $\mathrm{Br}$ and $\mathrm{Cl}$ atoms are on opposite sides of the double bond. The CIP descriptor of this isomer is

On each carbon the halogen outranks $\mathrm{H}$, so the two higher priority groups are $\mathrm{Br}$ and $\mathrm{Cl}$. They lie on opposite sides, so the isomer is $E$ (trans-like). $R$/$S$ do not apply to a double bond.