What is the meaning of uppajjati versus upapajjati?

In this post Ven. Sujato wrote,

Throughout, Ñāṇānanda refers to the reading anuppanno. This is the mainline reading of the PTS edition, and is presumably supported by the Sinhala manuscripts. The Burmese texts have anupapanno. The difference is that, as I discussed in a previous post, upapanna always refers to rebirth, whereas uppanna has a more general meaning. Nevertheless, in this context it is clear that rebirth is the topic, so the difference is immaterial. The related passages I quote below support the reading upapanna so I use that.

That's compatible with AN 3.59 which uses uppanna for the arising of "knowledge":

Ignorance is destroyed and knowledge has arisen; darkness is destroyed and light has arisen, as happens for a meditator who is diligent, keen, and resolute.
avijjā vihatā, vijjā uppannā; tamo vihato, āloko uppanno yathā taṃ appamattassa ātāpino pahitattassa viharato.

In AN 8.29 uppanna is used for the arising of a Tathagata in the world, but (in the next paragraph) upapanna for a person having been reborn in hell:

Firstly, a Realized One has arisen in the world—perfected, a fully awakened Buddha, accomplished in knowledge and conduct, holy, knower of the world, supreme guide for those who wish to train, teacher of gods and humans, awakened, blessed. He teaches the Dhamma leading to peace, extinguishment, awakening, as proclaimed by the Holy One.
Idha, bhikkhave, tathāgato ca loke uppanno hoti arahaṃ sammāsambuddho vijjācaraṇasampanno sugato lokavidū anuttaro purisadammasārathi satthā devamanussānaṃ buddho bhagavā, dhammo ca desiyati opasamiko parinibbāniko sambodhagāmī sugatappavedito;

But a person has been reborn in hell.
ayañca puggalo nirayaṃ upapanno hoti.

Piya Tan (in his description of Appassuta Sutta (AN 4.6) translates uppanna as "progress" or "spiritual progress".

That would be ironic if true, because "spiritual progress" may imply the opposite of "rebirth" (ironic that the dictionary definition of both include "rebirth").

But I think it's not generally true i.e. that specific meaning (spiritual progress) is only applies what it's applied to in this sutta.

I'm confused too though, by this. Below is Piya's rendition of AN 4.6. Note that:

• The Pali is given as uppanno
• The footnote says it's sometimes confused with upapanno

In contrast, below is Ven. Sujato's rendition of AN 4.6. Note:

• The Pali is given as upapanno (contradicting the Pali in Piya's version)
• The translation "get the point of learning" (which is perhaps more literally "learning comes into existence") contradicts Ven. Sujato's own comment that upapanna always refers to rebirth, so I don't know what's happening there

What four? A person may have:
Katame cattāro?

1. Little learning and not get the point of learning.
2. Little learning but get the point of learning.
3. Much learning but not get the point of learning.
4. Much learning and get the point of learning.
   Appassuto sutena anupapanno, appassuto sutena upapanno, bahussuto sutena anupapanno, bahussuto sutena upapanno.

I don't know anything about why there are different versions of the Pali, except that apparently there are. Maybe it's what Ven. Sujato was talking about in the first quote:

Ñāṇānanda refers to the reading anuppanno. This is the mainline reading of the PTS edition, and is presumably supported by the Sinhala manuscripts. The Burmese texts have anupapanno.

See also the quote appended to the OP:

it is almost impossible to distinguish between upap˚ and upp˚ in the Sinhalese writing

Confusing! Maybe you can derive some kind of clue from other bits of this answer though.

The use of "upapajjati" in MN 72 is very interesting and appears not so straightforward.

Yes MN 72 seems to use both words.

The instances of upapajjati are translated "reborn". And assuming you agree with this comment ...

In addition, another shorting coming of your post is you spend time analysing a term "ditthigata" which is spoken by an outsider, as though its meaning is the same as held by the Buddha. Since the outsider is ignorant of Dhamma, one must be careful in imputing, as you did, that their meaning of a word is the same as the Buddha.

... you'll find it significant that it was "the outsider" who first used that word.

And at the end of MN 72 there's anuppādadhammā (I think that's the same word as uppajjati) applied to each of the five aggregates -- saying that because form etc. don't "arise" then there's no question of being "reborn".

And, to "discuss the construction of the Pali words" -- according to this dictionary:

• One is upapajjati : (upa + pad + ya) to be reborn in; rises.
• And the other is uppajjati : (u + pad + ya) to be born; arises.

'Upapajjati' is for the most part found in suttas about results of kamma. It was also mentioned how it is found in the Appassuta Sutta (AN 4.6) about "progression" in learning/study.

'Uppajjati' is found unrelated to results kamma, most commonly as:

Eye consciousness arises dependent on the eye and sights.

Cakkhuñca paṭicca rūpe ca uppajjati cakkhuviññāṇaṃ

MN 148

Interestingly both words are found in the one place in MN 148:

If anyone says, ‘the eye is self,’ that is not tenable (12).

‘Cakkhu attā’ti yo vadeyya taṃ na upapajjati.

The arising and vanishing of the eye is evident,

Cakkhussa uppādopi vayopi paññāyati.

so it would follow that one’s self arises (13) and vanishes (14).

Yassa kho pana uppādopi vayopi paññāyati, ‘attā me uppajjati ca veti cā’ti iccassa evamāgataṃ hoti.

That’s why it’s not tenable to claim that

Tasmā taṃ na upapajjati:

the eye is self.

‘cakkhu attā’ti yo vadeyya.

So the eye is not self

MN 148

Piya Tan comments however appears to dispute the use of 'uppajjati' at 13:

12 “This is not fitting,” taṁ na upapajjati (lit “this does not arise”), but here Comy glosses as na yujjati, ‘it is not applicable; this is not the case/meaning” (MA 5:100).

13 “Rises,” upapajjati (PTS has uppajjati, which is erroneous), normally means “he reappears, is reborn,” but also has a special usage in logic to mean, “to be tenable, to be acceptable,” as it does here. (M:ÑB 1362 n1330)

14 “Passes away,” veti (from vi + √I, “to go”  eti) (Skt vyeti), “he goes away, disappears, wanes” (M 3:282; S3:135; A 2:51; J 3:154). Comy glosses as vigacchati nirujjhati (“he disappears, ceases”) (MA 5:100).

I disagree with Piya Tan's dispute about "uppajjati" at 13 and agree with the answer of Guru Andrei Volkov, from which I have concluded "upapajjati" to mean: "follows" or "proceeds" or "progresses".

Thus, to me, it means:

If anyone says, ‘the eye is self,’ that does not follow/proceed/conclude [from that statement].

MN 148

---

Because of undertaking such deeds, when their body breaks up, after death, they follow on/proceed into a place of loss, a bad place, the underworld, hell.

So tena kammena evaṃ samattena evaṃ samādinnena kāyassa bhedā paraṃ maraṇā apāyaṃ duggatiṃ vinipātaṃ nirayaṃ upapajjati.

MN 135

---

Little learning but follow to/proceed to the point of learning.

appassuto sutena upapanno

AN 4.6

---

While not the words of the Buddha, MN 72 below:

But Master Gotama, when a mendicant’s mind is freed like this, where do they proceed to/go to?

Evaṃ vimuttacitto pana, bho gotama, bhikkhu kuhiṃ upapajjatī ti?

‘They proceed/they go’ doesn’t apply, Vaccha.

“Upapajjatīti kho, vaccha, na upeti”.

Well then, do they not proceed/not go anywhere?

Tena hi, bho gotama, na upapajjatī ti?

‘They don't proceed/don't go’ doesn’t apply, Vaccha.

“Na upapajjatīti kho, vaccha, na upeti”.

MN 72

Woohoo! Buddha's reply in MN 72 explained below:

Tatrāpāhaṃ, bhikkhave, neva āgatiṃ vadāmi, na gatiṃ, na ṭhitiṃ, na cutiṃ, na upapattiṃ; appatiṭṭhaṃ, appavattaṃ, anārammaṇamevetaṃ. Esevanto dukkhassā”ti.

Here, bhikkhus, I say there is no coming, no going, no staying, no deceasing, no uprising. Not fixed, not movable, it has no support. Just this is the end of suffering..

Ud 8.1