The meaning of
"IN THE SEEN, THERE WILL BE MERELY THE SEEN"
Please refer to this essential visual aid all along.
Let's see what the Bāhiya sutta (ud.1.10) has to say - the familiar Bāhiya sutta, to which one should prefer the Māluṅkyaputta sutta (SN 35.95) - in which the terms are identical; and which has, moreover, a quite reliable parallel in the Āgamas (SA 312).
The most important part of that Māluṅkyaputta sutta; which has the same formulation in its SA parallel, is the following:
When, Māluṅkyaputta, regarding things seen, heard, sensed, and cognized by you, in the seen there will be merely the seen, in the heard there will be merely the heard, in the sensed there will be merely the sensed, in the cognized there will be merely the cognized, then, Māluṅkyaputta:
Ettha ca te, mālukyaputta, diṭṭhasutamutaviññātabbesu dhammesu diṭṭhe diṭṭhamattaṃ bhavissati, sute sutamattaṃ bhavissati, mute mutamattaṃ bhavissati, viññāte viññātamattaṃ bhavissati. Yato kho te, mālukyaputta, diṭṭhasutamutaviññātabbesu dhammesu diṭṭhe diṭṭhamattaṃ bhavissati, sute sutamattaṃ bhavissati, mute mutamattaṃ bhavissati, viññāte viññātamattaṃ bhavissati;
You will not be ‘by that.’
When, Maluṅkyaputta, you are not ‘by that,’
then you will not be ‘in that.’
When, Maluṅkyaputta, you are not in that,’
then you will be neither here (nevidha - aka this kama world), nor there (huraṃ - aka the rūpa, world, viz. nāmarūpa nidāna), nor in between the two.
Tato tvaṃ, mālukyaputta, na tena.
Yato tvaṃ, mālukyaputta, na tena;
ato tvaṃ, mālukyaputta, na tattha.
Yato tvaṃ, mālukyaputta, na tattha;
tato tvaṃ, mālukyaputta, nevidha, na huraṃ, na ubhayamantarena.
To put it simply:
"In reference to the seen, there will be only the seen", just means that what is seen should not be considered as "ours".
Neither "I am this", nor "this is mine".
There is a slight nuance to be apprehended though.
The "there", the huraṃ (viz. the nāmarūpa nidāna & its khandhas), is linked to its various representations in the external fields of sensory experiences of the saḷāyatana nidāna.
When, for instance, it is said: "in the heard there will be merely the heard," it means that a form in nāmarūpa nidāna can have a guitar as its rūpa; and a sensory representation as the "sound" of it.
What is to be experienced is merely the sound, with a sense-consciousness gone through a purified mano (parisuddhena manoviññāṇena - MN 43). That is to say, a sense-consciousness that comprehends that: neither "I am this (sound)", nor "this is mine".
Restraining the indriyas ("keeping guard over the doors of the powers (faculties) > Indriyesu guttadvārā"), can't eradicate them totally.
In AN 6.55, Buddha states that nonetheless, they can be brought to their plain, flat, regular, viz. "normal" level.
Again, this is what is meant by "in the seen, only the seen".
That is to say, looking at something, knowing that it is not "yours" - and restraining the indriyani; so that the effect of these indriyani on the āyatanani, will not trigger sensory experiences that one could crave for and appropriate.
One has to realize that one is neither "in form", nor "apart from" form. (See Here & here). Therefore, one cannot experience without dealing with the external fields of sensory experience. However, one can restrain the indriya.
Then SN 35.87 becomes more meaningful:
Therefore, friend Channa, this teaching of the Blessed One is to be constantly striven after with the mano:
"For one who is clinging, there is the wish to act; for one who is not clinging, there is no wish to act. When there is no wish to act, there is tranquility (confidence); when there is tranquility, there is turning away; when there is turning away, there is no obtaining & non-obtaining; when there is no obtaining & non-obtaining, there is no removing from (namarūpa), and coming forth (in satta); when there is no removing from (namarūpa), and coming forth (in satta) [lit. leaving one world and coming to another = the process of phassa], there is neither here, nor there, nor in between the two."
Tasmātiha, āvuso channa, idampi tassa bhagavato sāsanaṃ niccakappaṃ sādhukaṃ manasi kātabbaṃ:
"Nissitassa calitaṃ, anissitassa calitaṃ natthi. Calite asati passaddhi hoti. Passaddhiyā sati nati na hoti. Natiyā asati āgatigati na hoti. Āgatigatiyā asati cutūpapāto na hoti. Cutūpapāte asati nevidha na huraṃ na ubhayamantarena."
Read also SN 45.246