This point is also made by Norman Swartz and Raymond Bradley.
The two are less likely to be confused in ordinary language than subjunctive and epistemic possibility as there are some important differences in the logic of subjunctive modalities and deontic modalities.
In particular, subjunctive necessity entails truth: if people logically must such and such, then you can infer that they actually do it.
There are several different types of subjunctive modality, which can be classified as broader or more narrow than one another depending on how restrictive the rules for what counts as "possible" are.
Some of the most commonly discussed are: Similarly David Lewis could have taken a degree in Economics but not in, say, Aviation (because it was not taught at Harvard) or Cognitive Neuroscience (because the so-called 'conceptual space' for such a major did not exist).