When examining necessity, ask whether a feature is inseparable from the subject's essence or from some specified condition of its external existence.
Essentially necessary: four's evenness. Four cannot remain four while becoming odd.
Necessary relative to existence: a trait may be inseparable under a given condition of a subject's actual existence without entering the definition of its essence.
The source distinguishes these types of necessity. Most importantly, a feature's being necessary does not automatically make it a defining constituent.