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README.md

@@ -69,6 +69,16 @@ Index-based, shared across all PIC-Flow ablation runs:
 | val | 2,250 |
 | test | 2,250 |
 
+> **Note on the Hugging Face dataset viewer.** The viewer at the top of this page
+> labels every shard as "train" because it auto-detects splits from filename
+> patterns (`train-*`, `test-*`, `validation-*`). This dataset uses a different
+> convention: splits are encoded **per sample** in `shards/index.json` (each entry
+> has a `split: "train" | "val" | "test"` field), and a single `.npz` shard can
+> contain samples from any of the three splits. The PIC-Flow dataloader
+> ([`Model/dataset.py`](https://github.com/Rizzo-Integrated-Photonic-Systems-Lab/PIC-Flow/blob/main/Model/dataset.py))
+> reads `index.json` and partitions samples accordingly. The 18,000 / 2,250 / 2,250
+> split is what the dataloader actually serves — the viewer label is cosmetic.
+
 ## How to download
 
 ```bash