What determines the addition of the suffixes "tion" and "sion"?

Why does the B in the word absorb change to P when the suffix "tion" is added?

Absorb + tion = absorption and not absorbtion.

The answer to this question is here:

Question: Why does the B in absorb change to a P in absorption?

Answer: Too long but here is the main part:

Voicing Assimilation is the technical term for what happened here.

This was an existing question and answer but my question is different from that one so it's not a duplicate.

My question is:

*Why does the B change to P in the root word but the suffix remains the same. Why didn't the suffix "sion" attach to the word absorb? It could have been absorbsion.

If we have to keep the alternating sounds the same, then B and zh (of sion) are both voiced.

In most words, the sounds in the root words remain the same and the suffix is changed.

Is there any rule that determines the addition of "tion" and "sion"? When should we use "tion" and when "sion"?

The distribution of -sion and -tion in English is primarily historical, not rule-based: words that were spelled with sio(n) in Latin continue to be spelled with s, while words that were spelled with tio(n) in Latin are spelled with t (in the past, the letter c was a common alternative to t in words like this).

In Latin, -sio, -sion- is the form that the suffix -tio, -tion- regularly took when it combined with a preceding “dental” consonant (Proto-Indo-European *t, *d or *dh, which generally correspond to Latin t and d). In a few contexts, Proto-Indo-European *dh turned into Latin b, as in iubeo, which has a related noun iussio. But the b in absorbeo does not come from *dh. Since the stem of absorbeo never ended in a dental consonant, it isn't expected to take the -sio variant of the suffix -tio.

The -sio variant was expanded to some words that didn’t originally have a dental consonant, but that didn’t happen in the case of absorptio.

In English, -tion is a more common termination than -sion, so there is not much pressure within English to replace -tion with -sion: in fact, -tion spellings have expanded compared to Latin in the case of certain words that originally had -xio(n) such as reflexion and connexion. In terms of phonetics, there are many other words with /pʃ/; e.g. reception, interception, inscription, corruption; the same can’t be said for /bʒ/.