Third level appears as soon as you copy, say, 1500 points from cached chunks of 1000 points.
Yes, I’m talking about annotations as one of several signal types, known as labels or segmentation. So, they are indeed output of some algorithm. Especially if we are talking about tebadytes of data, because you cannot manually annotate terabytes in a reasonable amount of time. You can only review a small portion of automatically labeled/annotated data.
I’ts not about HDF5 limitations, because there are different similar formats like Arrow, Zarr, Exdir, TileDB, N5, Z5, etc.
If Onda is a layer above already stored files of different formats (something like file database), then it is more about mapping different formats with software to read from them, mapping data from files to metadata, and taking special attention to metadata structures that are added on top of those files. Here are some thoughts on working with metadata: Do you use some file database with tagging / multiple grouping functionality?