Was is the difference between "uparujjhati" & "nirujjhati"?

My answer is actually a response or comment to Andrei Volkov's highly illuminating answer; which reflects my interpretation and application of Andrei's answer.

When dependent origination ends forever (SN 12.2) or craving ends forever (SN 56.11), without ever arising again, it is called "asesa-virāga-nirodhā".

So in DN 11, the guy asks: "where do the four elements aparisesā nirujjhanti". Here, "aparisesā" might mean "totally not remaining" (an + pari + sesa = completely not remaining) ????.

The Buddha replies this is a wrong question. The Buddha then replies in luminous consciousness, the worldly discriminations are subjected to asesaṃ uparujjhatī.

While "asesam" appears to mean "nothing left over", "uparujjhati" does not have the same meaning as "nirodha" or "nirujjhanti" because it is only in luminous consciousness that these discriminations completely cease.

In other words, even in ignorance-free craving-free enlightened ordinary consciousness, such as a Buddha or Arahant in a conventional mode (such as the Buddha telling a monk to do walking meditation over the "length of 30 short steps" or to know "long breathing" & "short breathing"), these discriminations (of long, short, fine, coarse, beautiful, ugly, naming a form) can return.

Thus these discriminations are "uparujjhati" because, unlike the cessation of ignorance for an Arahant, these conventional discriminations can return.

In other words, similar to the prefix "upa" in "upapajjati", the cessation of the discriminations is an "offshoot" of or "rests" upon the luminous consciousness (rather than is an outcome of the permanent or remainderless cessation of ignorance, which is "nirodha" or "nirujjhanti").

Thus, as Bhikkhu Sujato said: Nibbana is not viññāṇa. Really, it just isn’t.

According to the PTS Pali-English dictionary entry on the prefix upa-:

Upa -- [Vedic upa; Av. upa on, up; Gr. u(po/ under, u(pe/r over; Lat. sub fr. *(e)ks -- upo; Goth. uf under & on; Ohg. ūf = Ags. up = E. up; Oir. fo under. See also upari] prefix denoting nearness or close touch (cp. similarly ā), usually with the idea of approach from below or rest on top, on, upon, up, by. -- In compn. a upa is always contracted to upa, e. g. devūpaṭṭhāna, lokûpaga, puññûpatthambhita. -- Meanings: (1) (Rest): on upon, up -- : ˚kiṇṇa covered over; ˚jīvati live on (cp. anu˚); ˚tthambhita propped up, sup -- ported; ˚cita heaped up, ac -- cumulated; ˚dhāreti hold or take up; ˚nata bent on; ˚nissaya foundation; ...

According to the PTS Pali-English dictionary entry on rujjhati:

Rujjhati [Pass. of rundhati] to be broken up, to be destroyed J iii.181 (pāṇā rujjhanti; C. expls by nirujjhati). Cp. upa˚, vi˚.

Uparujjhati means "upon ceasing" with "upa-" meaning "upon" or "on", more or less.

For example, from SN 56.22:

ceases with nothing left over.
asesaṃ uparujjhati.

This means nothing left UPON ceasing. "asesa" means nothing remaining.

According to the PTS Pali-English dictionary entry on the prefix ni-

Ni˚ [Sk. ni -- & nih -- , insep. prefixes: (a) ni down=Av. ni, cp. Gr. neio/s lowland, nei/atos the lowest, hindmost; Lat. nīdus (*ni -- zdos: place to sit down=nest); Ags. nēol, nider=E. nether; Goth. nidar=Ohg. nidar; also Sk. nīca, nīpa etc. -- (b) niḥ out, prob. fr. *seni & to Lat. sine without]. ...

"Nirujjhati" means "breaks down" I think with "ni-" meaning "down", more or less.

From SN 12.62:

But that which is called ‘mind’ or ‘sentience’ or ‘consciousness’ arises as one thing and ceases as another all day and all night.
Yañca kho etaṃ, bhikkhave, vuccati cittaṃ itipi, mano itipi, viññāṇaṃ itipi, taṃ rattiyā ca divasassa ca aññadeva uppajjati aññaṃ nirujjhati.

Here, the mind rises UP as one thing (uppajjati) and breaks DOWN as another (nirujjhati).

The difference is primarily in upa, which is used as an intensifier to designate the circumstance of cessation. Since it is an intensifier, we are led to read "obliteration" rather than "fading away" as for "nirujjhati", where the "ni-" is providing emphasis by repetition of meaning (i.e., ni- + rujjhati).

One can also feel the meaning of the prefixes "upa-" and "ni-" when performing physical actions:

• Say "upa" when lifting or moving a heavy object. The breath is stopped briefly for exertion.
• Say "ni" when putting that heavy object down. The breath is exhaled continuously and let go.