The two-letter symbols A1 etc. are morphophonemes, i.e. abstractions over sets of phonemes alternating with each other, e.g. in Umlaut. They are used as indicated below:
[Todo: rewrite this in a general way, and not only to nouns. Link it to umlaut erc., in order to make the explanation cover the verbs and adjectives as well.]
Explanations: A1 = illative singular vowel A2 = ending vowel in words like 'maana' A3 = second vowel in words like 'daktere' A4 = first vowel in words like 'jeptsie' E1 = first vowel in words like 'sjiellie' U2 = ending vowel in words like 'nïejte' I1 = ie in all cases except ending vowel in words like 'gåetie' I2 = ending vowel in words like 'gåetie' I3 = first vowel of many case endings I4 = ending vowel on all nouns on three syllables, like 'gierehtse' O1 = oe in all cases except ending vowel in words like 'bearkoe' O2 = ending vowel in words like 'bearkoe' U1 = first vowel in words like 'njueslie' Æ = first vowel in words like 'klihtie' Å1 = first vowel in words like 'gullie' Å2 = first vowel in words like 'gaevlie' Å3 = first vowel in words like 'gåetie' when umlauting in plural Å4 = first vowel in words like 'gåetie' when not umlauting in plural D1 = possible doubling of preceeding consonant ... more to come for sure ;)
The main challenge is to rewrite the rules in order to make them work for verbs as well as for nouns.