The explanation for Arūpadhātu says this:
Arūpadhātuyā upapattikkhaṇe katamāni dve āyatanāni pātubhavanti? Manāyatanaṁ, dhammāyatanaṁ— arūpadhātuyā upapattikkhaṇe imāni dve āyatanāni pātubhavanti.
What is the difference between manāyatanaṁ and dhammāyatanaṁ?
