Cis/trans naming is the older, more intuitive system: cis means the two "main" substituents sit on the same side of the double bond, and trans means they sit on opposite sides. It works perfectly well for simple cases where it's visually obvious which substituent on each carbon counts as the important one to compare — typically when each double-bond carbon has exactly one hydrogen and one other, larger group.

E/Z naming, by contrast, doesn't rely on visual size or intuition at all — it uses the rigorous CIP priority rules you learned in the E/Z Isomerism lesson to rank the two groups on each carbon, then reports whether the higher-priority groups sit on the same side (Z) or opposite sides (E). Because it's based on a strict, unambiguous ranking procedure rather than a visual judgment call, E/Z works correctly on every alkene, no matter how many different substituents are attached or how similar they look in size.

Cis/trans naming simply has no defined answer once a double bond carbon carries two different non-hydrogen substituents (a trisubstituted or tetrasubstituted alkene) — there's no longer an obvious single 'main group' on that carbon to compare against the other carbon's groups, so the visual cis/trans judgment call breaks down entirely. E/Z naming has no such limitation, since CIP priority ranking works identically regardless of how many substituents are present or how similar in size they might look.

This is exactly why the hub's guidance is so direct: default to E/Z assignment as your standard practice, reserving cis/trans only for situations where a question specifically asks for it by name, or for describing simple, well-known compounds informally (like referring to 'cis-2-butene' in casual conversation, even while its formal IUPAC name uses the Z descriptor).