What is the "meditation on emptiness" in MN 121? What is the "meditation on emptiness" in MN 121?

What does "emptiness" refer to in this sutta?

Also, what does "oneness dependent on the perception of ..." mean in this sutta?


  “Indeed, Ānanda, you properly heard, learned, attended, and remembered
  that. Now, as before, I usually practice the meditation on emptiness.
  
  Consider this stilt longhouse of Migāra’s mother. It’s empty of
  elephants, cows, horses, and mares; of gold and money; and of
  gatherings of men and women. There is only this that is not emptiness,
  namely, the oneness dependent on the mendicant Saṅgha. In the same
  way, a mendicant—ignoring the perception of the village and the
  perception of people—focuses on the oneness dependent on the
  perception of wilderness. Their mind becomes eager, confident,
  settled, and decided in that perception of wilderness. They
  understand: ‘Here there is no stress due to the perception of village
  or the perception of people. There is only this modicum of stress,
  namely the oneness dependent on the perception of wilderness.’ They
  understand: ‘This field of perception is empty of the perception of
  the village. It is empty of the perception of people. There is only
  this that is not emptiness, namely the oneness dependent on the
  perception of wilderness.’ And so they regard it as empty of what is
  not there, but as to what remains they understand that it is present.
  That’s how emptiness is born in them—genuine, undistorted, and pure.
  
  ......
  
  Whatever ascetics and brahmins enter and remain in the pure, ultimate,
  supreme emptiness—whether in the past, future, or present—all of them
  enter and remain in this same pure, ultimate, supreme emptiness. So,
  Ānanda, you should train like this: ‘We will enter and remain in the
  pure, ultimate, supreme emptiness.’ That’s how you should train.”
What is the "meditation on emptiness" in MN 121?


The process of gradually flushing-out of various subjects from gross to subtle as detailed in paragraphs 4 - 12 of MN 121


  What does "emptiness" refer to in this sutta?


Per Ven. Bodhi's note: SunnataVihara, the fruition attainment of voidness/sunnataphala samapatti, fruition attainment of arahantship that is entered by focusing upon the void aspect of Nibbana.


  Also, what does "oneness dependent on the perception of ..." mean in this sutta?


Ven. Thanissaro's more straightforward rendering: "attends to the singleness based on the &lt;objects...&gt;", which Ven. Bodhi cited Comy's explanation:


  MA: He attends to the perception of forest dependent on the single forest itself, thinking: “This is a forest, this a tree, this a mountain, this a grove.” In the next sentence I read with BBS and SBJ adhimuccati, as against PTS vimuccati.