Meaning of Kammaṭṭhāna

You probably have seen this already, but it’s interesting that the tika gives the gloss (to Vism Ch 4 #29, regarding ‘nimittam ganhantena bhāvetabbam’)

“The dawn colour that is there in the kasina should not be thought about, though it cannot be denied that it is apprehended by eye-consciousness. That is why, instead of saying here ‘should not be looked at,’ he says that it should not be apprehended by reviewing. “
(Na vanno paccavekkhitabbo)

Ven Nanamoli, trans.

Vism goes on to say, (29) “attention should be given by setting the mind on the (name) concept as the most outstanding mental datum, relegating the colour to the position of a property of its physical support. “

Taking this sutta into consideration:

Master Gotama, these five faculties have different scopes and different ranges, and don’t experience each others’ scope and range. What five? The faculties of the eye, ear, nose, tongue, and body. What do these five faculties, with their different scopes and ranges, have recourse to? What experiences their scopes and ranges?”

“Brahmin, these five faculties have different scopes and different ranges, and don’t experience each others’ scope and range. What five? The faculties of the eye, ear, nose, tongue, and body. These five faculties, with their different scopes and ranges, have recourse to the mind. And the mind experiences their scopes and ranges. -SN 48.42

…it may be risky to assume direct access to the five senses without considering the position of mind (mano), also a sense base. Also important to note that both mano and citta have a position in what is being described in MN 20 - mano as far as attention is concerned, and citta in regards to the composure that results from successfully avoiding the unwholesome thinking.

But to answer your question best I can…based on MN 20, it seems the nimitta is evident when thinking connected with greed, hate and delusion is understood to be unwholesome - or in the very least - an issue to mitigate. That recognition would apply to a state of mind being “not wanted”: mind with greed, hate and/or delusion. Safe to say this is cetasika (mental), having to do with the condition of citta as a result of mano (SN 48.36).

Only when those things have been understood as not worth tolerating, would there be a red flag of sorts. Think about it. If one is making routine use of those unwholesome roots, the likelihood of them being recognized as unwholesome, and not beneficial, is less. But, when they have been thoroughly understood as exactly that, there will be more and more of chance that a discrepancy will be picked up, and unwholesome thinking cut off.

Again, this would all stem from previous reflection, and the development of virtue and restraint, whereby the principle of avoiding actions that are not beneficial already has a firm basis in the experience. Without that skill set established on the level of body and speech, there is little chance of introducing such limits/confines at the level of thinking. So, whatever the “meditation object” or “place of action” (kammaṭṭhāna), it seems some serious strides must be made for that mental work to be practically available. It becomes difficult to reconcile the use of any perception or reference of mindfulness for the purposes of purification if the lifestyle has not been thoroughly altered already. Apologies if I’m stressing this point too hard.