I do not study these works but i believe he is making a reference to everything nama always being tied to vinnana whereas form isn't.
"Feeling, perception, & consciousness are conjoined, friend, not disjoined. It is not possible, having separated them one from another, to delineate the difference among them. For what one feels, that one perceives. What one perceives, that one cognizes. Therefore these qualities are conjoined, not disjoined, and it is not possible, having separated them one from another, to delineate the difference among them."
"Discernment & consciousness are conjoined, friend, not disjoined. It's not possible, having separated them one from the other, to delineate the difference between them. For what one discerns, that one cognizes. What one cognizes, that one discerns. Therefore these qualities are conjoined, not disjoined, and it is not possible, having separated them one from another, to delineate the difference between them."
Likewise contact is always associated with either of 6 types of consciousness.
There are 3 aggregates that are conjoined with consciousnesses; perceptions, feelings and sankhara which generates consciousness as conscioussness or form as form & so on.
Forms aggregated are not conjoined with consciousnesses aggregated, because form does not always accompany consciousness, form is not always generated.
Four aggregates have objects. Forms aggregated do not have an object.
In nama you will also see attention & intention.
Attention attends to, it brings into being this or that, appropriate or inappropriate dhammā.
Intention refers to the 6 classes of genesis [sankhara].
why is sañña given some kind of precedence in its inclusion within the category ahead of Phassa?
In as far as there is anything, there contact can be delineated.
Where contact is delineated, there a particular consciousness implicated in that very contact can be delineated.
Where consciousness is delineated, it is said to be conjoined with percipience & feeling.
In short there are two types of contact, with form & without form, but there is always sanna in as far there is consciousness delineated, due to their semantic conjoinment.
The states where contact is without form is that which said to be known with the purified intellect-consciousness divorced from the five [sense] faculties [form].
"Friend, with the purified intellect-consciousness divorced from the five faculties the dimension of the infinitude of space can be known [as] 'infinite space.' The dimension of the infinitude of consciousness can be known [as] 'infinite consciousness.' The dimension of nothingness can be known [as] 'There is nothing.'
When Ven. NV writes;
Phassa is included in nāma since nāma, in specifying saññā, necessarily specifies the pair of āyatanāni ('bases') and kind of viññāna involved
I believe he is referring to contact being a meeting of the three, eg;
1.Form which is the eye + 2.Form which is visible by the Eye + 3.Eye-consciousness.
Here 1 & 2 are bases which are form.
If intellect is divorced from form the contact is;
- Intellect + 2. Ideation + 3. Intellect-consciousness.
Ideation here must also be not such that is tied to form.
Here 1 & 2 are bases which are not-form.
Either way there are two bases and a respective consciousness.