What does the manas's self-attachment cling to?

The manas's grasping at self is innate, and is called innate self-grasping (sahaja-ātma-grāha). It mainly grasps at the five aggregates and eighteen realms as "self" and "belonging to self," as having real functions, grasping at self, at person, at sentient beings, at life span, grasping at the realms of the six sense objects, grasping at the functions of the six faculties, grasping at the knowing and seeing of the six consciousnesses, grasping at feeling, perception, volitional formations, and consciousness, and grasping at the physical body. As for the attachment to other, subtler dharmas, it is named dharma-grasping—grasping dharmas as real, as belonging to self, as having actual functions. Of course, when one does not yet know or understand many dharmas, one has no way to grasp at them. As the degree of one's cultivation deepens continuously, and the dharmas one understands become more and more numerous and ever subtler, the dharma-grasping in the mind will keep appearing and strengthening, and one must engage in continuous contemplative practice (vipaśyanā-caryā) to subdue, remove, and eradicate it.

The mind consciousness also has grasping, only it is not as severe as that of the manas; it is a grasping at sense objects that arises only when one is in the midst of those objects. The more one contacts the realms of the six sense objects, the more and heavier the grasping becomes; this is called discriminating self-grasping (parikalpita-ātma-grāha). The mind consciousness's discriminating self-grasping is intermittent, appearing and disappearing; when the mind consciousness ceases, the grasping ceases; when the mind consciousness understands the principle, the grasping will be subdued or perish. The mind consciousness's grasping at sense objects is lighter than the manas's grasping, not so stubborn—it can be relinquished the moment one says "let go," and perhaps after a night's sleep it is forgotten. The manas's grasping, however, cannot be forgotten even after a lifetime of sleep, cannot be eliminated, is stubborn to the extreme, and at times, the more one goes against it, the more stubborn it becomes.