Understanding the Mental Objects (Dhammārammaṇa) may helps:
In the CMA (A Comprehensive Manual of Abhidhamma) page 136 it gives the details of Mental Object as below:
Mental object is sixfold: Each of the first five objects can be cognized in any of these ways: (1) through its own respective sense-door process; (2) through a mind-door process; and (3) by the process-freed cittas occurring in the roles of rebirth-linking, bhavaṅga, and death. Mental objects—the objects of the sixth class—cannot be cognized at all through a sense-door process. They can be cognized only by the cittas of a mind-door process or by the process-freed cittas that occur independent of the sense doors.
Six kinds of objects fall into the category of mental object (dhammārammaṇa).
1. Sensitive matter (pasādarūpa) is the sensory receptive substance in the five sense organs; it is fivefold, eye-sensitivity, ear-sensitivity, nose-sensitivity, tongue-sensitivity, and body-sensitivity. [All sensory inputs are resultant (vipāka).]
2. Subtle matter (sukhumarūpa) includes sixteen species of material phenomena enumerated below (VI, §6), among them the water element.
3. Citta is also a type of mental object. Though citta experiences objects, citta in turn can become an object. It should be noted that a citta in its immediacy cannot become its own object, for the cognizer cannot cognize itself; but a citta in an individual mental continuum can experience earlier cittas in that same continuum as well as the cittas of other beings.
4. The fifty-two cetasikas can also become objects of a mind-door process, as for example, when one becomes aware of one’s feelings, volitions, and emotions.
5. Nibbāna becomes the object of cittas occurring in the mental processes of noble individuals, both trainees and Arahants.
6. Concepts —the class of conventional realities, things which do not exist in the ultimate sense—also fall into the category of mental object.
With mettā, Seng Kiat