diff --git a/docs/RUGNUX.md b/docs/RUGNUX.md
index c15b6e3e..86fa063c 100644
--- a/docs/RUGNUX.md
+++ b/docs/RUGNUX.md
@@ -408,7 +408,7 @@ General:
 | Option | Description |
 | --- | --- |
 | `-o, --output-prefix <txt>` | Output file prefix (default: `output`) |
-| `-N, --threads <num>` | Number of worker threads (default: all hardware threads, with the per-image loop of `--mode mx` capped at 16 workers per GPU; an explicit value is used as given) |
+| `-N, --threads <num>` | Number of worker threads (default, and for any value ≤ 0: all hardware threads). Some stages take fewer, because past a point more workers make them slower: the per-image loop of `--mode mx` uses at most 16 per GPU unless `-N` was given a positive value, and first-pass spot finding and the beam-stop pre-scan have ceilings of their own that `-N` does not lift. Scaling, merging and the space-group search use the full count |
 | `-s, --start-image <num>` | First image to process (default: 0) |
 | `-e, --end-image <num>` | Last image to process (default: all) |
 | `-t, --stride <num>` | Process every *n*-th image (default: 1) |