What are these positive terms?

It's not clear to me how he uses the words "positive" and "negative".

The usual meaning of these words is something like, "negative is bad, positive is good", or perhaps, "negative is non-existent, positive is existent", but I don't think that's what he means.

I think the word is (like most "models" or "theories" are) an analogy -- and that the way he uses the word, or the meaning the word has for him, is the meaning implied by a photographic negative -- which was a contemporary meaning in his lifetime, but is obsolete or archaic now because we use digital photography instead of photographic film.

The reason I say that is because of his use of the word "negative" on this page:

Fundamental Structure >> Static Aspect

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... where the "negative" is an inverse (i.e. o becomes x and vice versa) like a photographic negative.

There's a more detailed example use his use of "positive" and "negative" on this page:

Shorter Notes >> Attā

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'Self' as subject can be briefly discussed as follows. As pointed out in PHASSA [b], the puthujjana thinks 'things are mine (i.e. are my concern) because I am, because I exist'. He takes the subject ('I') for granted; and if things are appropriated, that is because he, the subject, exists. The ditthisampanna (or sotāpanna) sees, however, that this is the wrong way round. He sees that the notion 'I am' arises because things (so long as there is any trace of avijjā) present themselves as 'mine'. This significance (or intention, or determination), 'mine' or 'for me'—see A NOTE ON PATICCASAMUPPĀDA [e]—, is, in a sense, a void, a negative aspect of the present thing (or existing phenomenon), since it simply points to a subject; and the puthujjana, not seeing impermanence (or more specifically, not seeing the impermanence of this ubiquitous determination), deceives himself into supposing that there actually exists a subject—'self'—independent of the object (which latter, as the ditthisampanna well understands, is merely the positive aspect of the phenomenon—that which is 'for me'). In this way it may be seen that the puthujjana's experience, pañc'upādānakkhandhā, has a negative aspect (the subject) and a positive aspect (the object). But care is needed; for, in fact, the division subject/object is not a simple negative/positive division. If it were, only the positive would be present (as an existing phenomenon) and the negative (the subject) would not be present at all—it would simply not exist. But the subject is, in a sense, phenomenal: it (or he) is an existing phenomenal negative, a negative that appears; for the puthujjana asserts the present reality of his 'self' ('the irreplaceable being that I am'). The fact is, that the intention or determination 'mine', pointing to a subject, is a complex structure involving avijjā. The subject is not simply a negative in relation to the positive object: it (or he) is master over the object, and is thus a kind of positive negative, a master who does not appear explicitly but who, somehow or other, nevertheless exists.[c] It is this master whom the puthujjana, when he engages in reflexion, is seeking to identify—in vain![d] This delusive mastery of subject over object must be rigorously distinguished from the reflexive power of control or choice that is exercised in voluntary action by puthujjana and arahat alike.

He seems to be saying that intention isn't a (positive) thing but is a instead structure created by or around the existence of some (real) thing.

Conversely (to answer your question), it's not that nāmarupa and viññana have positive and negative terms -- it's that he's saying that nāmarupa and viññana are positive terms, things which exist in their own right.

Instead of saying "... in the positive terms of nāmarūpa and viññāna", he might have said, "... in terms of nāmarūpa and viññāna which are 'a matter of positives'."

Incidentally this discussion of positive and negative puts me in mind of a line from the Tao Te Ching -- describing a house, it says, "it's the empty space in a house (i.e. the doors and windows and rooms) that make it useful". I think that's another example, the "empty space" being the "negative" created by or from the house's solid (existing, filled, positive) form.