There are 6 dhatu in all: patavi, apo, tejo, vayo, akasa, vinnana.
patavi, apo, tejo, vayo are in akasa. Therefore, akasa can be thought of as “where patavi, apo, tejo, vayo exist”.
Both akasa and vinnana are infinite. One can experience that (I have not) when one gets into the first two arupavacara jahna of Ākāsānancāyatana and Viññāṇañcāyatana.
Ākāsānancāyatana comes from Ākāsā + ananta + ayatana, which means it is “infinite spatial plane”. Plane does not mean a two-dimensional surface. I cannot think about a suitable English word; it just means “it is out there”.
Same for Viññāṇañcāyatana: “infinite Viññāṇa plane”.
What is Viññāṇa dhatu are Viññāṇa (kammic energies below the suddhashtaka stage) and nama gotta (just records without energy).
Sanna, Vedana, Sankhara just arise in one’s mind; as soon as they arise, they are gone. After that records of them remain as nama gotta in the Viññāṇa plane.
– For example, one can recall what happened yesterday while eating lunch: one recognized the food, tatsed, and generated sankhara about it. Those things one can recall.
– But if one got really attached to that food, and wants to eat the same next week, that “expectation” remains as an energy as a Viññāṇa.