One issue with the categories of MIT90s is that they are very broad, especially Individuals and roles. So within universities it is conventional to split that one into at least two parts: Staff (within the organisational boundary) and Students (in some sense customers). Each such part is called a subcategory.
Some experts want a further subcategory for senior management.
It is a moot point how other stakeholders should be handled within the MIT90s framework.
Some further guidance on subcategories (and on the metacategory concept) comes from reflecting on how to map the McKinsey 7S model into MIT90s - see the MIT90s methodology article for references to that.