thanks for bringing up the commentary. ;-) I have a look and it says,
neyyattha.m ... netabbattha.m ...: meaning should be offered.
niitattho ... kathitattho ...: meaning revealed.
So, PTS English is not far off to have "already explained" and "needing explanation" for niitattha and neyyattha respectively.
The commentary further comments that where the paramattha (anicca, dukkha, anatta) is not given, then it is neyyattha, but the fool would say that it is niitattha; and where the paramattha is given, the fool would treat it otherwise and say it requires further explanation.
I also have a look at Nettippakara.na commentary, in which the relevant section has,
neyyatthanti niddhaaretvaa gahetabbattha.m: meaning should be provided and taken.
niitatthanti yathaarutavasena ~naatabbattha.m: meaning should be known, (?) as if by the influence of the sound (?).
The verb neti can also mean: to understand, in addition to: to guide. However, the traditional views of the commentary suggest differently.
It is also interesting to note that both neyya and niita are derived from neti: neyya (gerundive) and niita (past participle).
--- In Pali@yahoogroups.com, Nina van Gorkom wrote:
> For (a) neyyattha suttanta, and (b) niitattha suttanta. I take
> both neyyattha and niitattha as adjectives.
> PTS PED lists neyyattha as the compound "neyya attha", i.e.
> meaning to be understood, and niitattha as the opposite. I also
> have a look under the entry attha, it says niitattha = primary
> meaning, literal meaning; neyyattha = secondary or inferred meaning.
> Putting these together, I would suggest "neyyattha suttanta" as
> "discourse with meaning to be understood, or requiring some
> interpretation", and "niitattha suttanta" as "discourse which can
> be understood literally, or not requiring any interpretation".
N: The PED point to the netti pakara.na commentary and this is similar to what you suggest: <One whose meaning is already guided? One whose meaning has yet to be guided>
Footnote: niitatthaa and neyyattha. From neti: to guide.
I do not take so much to PED's primary meaning and secondary meaning. Niitattha is of the past, already 'concluded' or explained. I would think that the meaning has been explained already, it does not need to be explained again.