Nāma Pannatis (Name concepts) are 6 fold.
- Vijjamāna Pannatti: Concepts about real (eg: pathavi, āpo )
- Avijjamāna Pannatti: Concepts about unreal (eg: person, atta)
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Therefore āpo, vāyo etc. are “Concepts about real” while time, space etc. are “Concepts about unreal”.
The name-concept “āpo” has a corresponding reality (paramattha) that it refers to. Therefore āpo is not an imagined dhatu.
I heard ven. Maggavihari describing āpo as cohesion or attraction or liquidity. Further he said that there are inner-kalapa cohesion and inter-kalapa cohesion.