How and why that rupa mentioned in the patthana gets its name.
yaṃ rūpaṃ nissāya manodhātu ca manoviññāṇadhātu ca vattanti, taṃ rūpaṃ manodhātuyā ca manoviññāṇadhātuyā ca taṃsampayuttakānañca dhammānaṃ nissayapaccayena paccayo.
~patthana
The (nissaya)vatthu or base of (manodhātu•manoviññāṇadhātu)hadaya is called hadayavatthu.
hadayassa vā manoviññāṇassa vatthu hadayavatthu.
Here manoviññāṇa is a collective term for manodhātu and manoviññāṇadhātu.