From Carnie's "Syntax: A Generative Introduction":
This notation is not explicitly explained. What does it mean? (I'll write my conjecture below)
Recall that T is defined as follows:
So I guess the first row in 17) means that the subcategory of T consisting of auxiliaries includes only non-modal and non-nonfinite verbs. The second row should mean that the subcategory of T consisting of modals includes only modal and non-nonfinite verbs. Is this a correct interpretation? It sort of makes sense to me.
But the third row is a little bit confusing. Is "to" a modal (since there is +modal mark)? Why isn't it included in "Modals" in 16) then?