NOTATIO

Thanks for the great explanation, Neera!

You are most welcome!

Here are two possible options:
(1) (2) The first one is hyper-clear, with complete string indicators and harmonic circles.
The second one—which I would expect coming from a non-string-player composer—only states the minimum necessary to avoid any misunderstanding. 0+circle = open string + harmonic, with one and only one option to play that correctly. 'III' under the D-A is unmistakable: 4th position, G-D strings.
'0' above D-C may appear misleading but it is again super-clear because there is no middle-C open string on cello so it must be the D.
In this last case, a 'II' under D-C may have not been enough because the middle-C is available in the D string in the previous position.

One question, Neera. Were I the composer and this not an educational work or edition of a standard work with editorial fingering and bowing, I'd only put in the one harmonic indication and assume that the rest was obvious to an experienced cellist. Isn't that the normal way to handle this when nothing special is being asked for?

Yes, John.
The harmonic would be needed because the fourth is not a friendly interval on the cello.
What would prompt players to play on the middle strings would be the PP.

So now the question is why this composer is putting in extra indications that are not usually inserted in an original, non-educational work.

No idea why, but he/she wouldn't be the first one to suggest specific strings to play a certain passage on.
Nothing special going on here, really
—exactly as, for pianists, it is nothing special to see missing rests and flipped stems in a bar because those rests are obvious! To everyone else that is a mistake, to pianists that is a given! Fascinating, right?

I guess we are fated to remain in the dark until we see some more of this music to understand the purpose of these extraneous indications. In my experience, composers don't waste time putting in fingering and such except for really good reasons. For example, all of Beethoven's piano fingering, of which there are only a few examples, is enlightening and adds something special that would not be clear otherwise.

There actually seems to be a kind of code of compositional and notational "ethics" among the composers that I have edited to keep things as simple as possible and not put in one more note or indication than is necessary.

Thank you once again, Neera, for sharing your experience and wisdom here. I've really learned a great deal from this post.

As far as the rationale behind why this composer chose to include this notation, it mostly like is because as Neera pointed out here:

The reason for not playing chords 2-4 this in strings I/II is that the A string sounds ~3x louder than the other strings with the same strength applied.

The passage where this notation appears is the B section, which is softer in dynamic and has a textural change. If it weren't notated as such, the cello would probably be too prominent for the composer's liking.

Thanks, czoller. Sorry for being so dense, Neera. Now I get it. So this is something special. For some reason I thought you were saying that avoiding the open strings was the normal way of handling these double stops, irrespective of dynamic indication.