This is an interesting section that distinguishes dhatus on the basis of whether they are subjective in nature, but its done in a roundabout way - by identifying those dhatus with objects. I can see the strategy here - a naive reification of our sense of subjectivity is a big part of why we suffer. To come out and say, "these dhatus are subjective in nature" might lend too much traction for those erroneous inclinations and confuse us. In an effort to cure ourselves we might go so far as to reject the whole dynamic, as the Buddha did in his ascetic phase before his awakening, which is not what we want to do. The subject-object formed consciousnesses are not the problem - its the false qualities that we emotionally attribute to them that is the problem. By identifying dhatus with objects, and the dependent nature with the subject, the emphasis is on the intrinsic relationship that defines the arising of subject with object. This view, I think, is more conducive to a neutral conception of these dhatus, as counseled by Sariputra:
https://www.accesstoinsight.org/tipitak ... .than.html
And a more elaborate sermon by the Buddha:
Seven Dhatus are identified as having objects - the six sensory consciousnesses and the dharmadhatu that is associated with the mind.
Its a little tricky to understand how dharmadhatu has an object. Mind, in contact with dharmadhatu, gives rise to mental consciousness. Here, they say mental objects have mental objects.
What are mental objects? They are in fact instances of mental consciousness that arose in the past (except for the ones that don't have objects).
The dharmadhatu without objects are discussed later in Chapter II. Without skipping ahead, it appears these are idiosyncratic notions of the Sarvastivadin that in Yogacara would be dharmadhatu with objects.
Next Vasubandhu identifies the dhatus that are "appropriated" or "non-appropriated". In short, by appropriated, we are describing dhatus as seized as aspects of self. Vasubandhu writes:
Appropriated dhatus appear to mean rupa (material form) that is perceived to be the basis of consciousness, ie. the vital parts of the body. Hair and nails, excrement, dandruff, snot, spit, etc. material that grows or sloughs or excreted off of us is not our body.What is the meaning of the expression "appropriated?" That which the mind and the mental states grasp and appropriate to themselves in the quality of a support is called "appropriated."... Matther that Abhidharma calls "appropriated," is called in common language, sacetana, or sensitive matter.
It could be argued that materialists are stuck on this perception and take the further step of asserting that consciousness is an epiphenomenon of matter. Vasubandhu touches on the relationship of mind and body in terms of our health characterizing it as reciprocal. In Western medicine, while there is growing appreciation of the function of mind in our health, mind is still generally discounted as a cause of good or bad health. For instance, an oncologist would hardly look at a person's mind as a cause of cancer while in Eastern medicine, mind would be considered as a primary cause.
Vasubandhu identifies 8 dhatus that are never appropriated - the 7 dhatus with objects (ie mental phenomena) and sound. The five present sense organs are appropriated, but the organs in the past and future, which are mere present conceptions, are not appropriated.
The distinction between appropriated and non-appropriated visible matter, odor, tastes and tangible objects, is interesting. When these things are qualities of "sensitive matter", they are appropriated, but when not, non-appropriated. This suggests a distinction in how modern, scientifically informed people will diverge in the perception of these things. For instance, we know that odor is the result of molecules becoming airborne and making contact with olfactory receptors in our nose. Odor, then is something that sloughs off of the body and is thus like dead skin or snot. Similarly, visible matter is actually light reflecting off of objects and reaching our retinal nerve. Taste is little more subtle, as are tactile objects.
The point is, in approaching this text, we must understand things as they describe them, not necessarily as we understand them to be. We are trying to step into the Sarvastivadin's conceptual universe.