diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 2eed38ee6..4fa4d8387 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -21350,9 +21350,12 @@ components:
           type: string
           minLength: 1
           maxLength: 60
-          description: "Apple's `HKUnit` string for `value`, captured for audit. The server does NOT convert from it: `value` is
-            read in the unit HealthLog stores the identifier in (`m` for walking distance, `kg` for body mass, `kcal`
-            for energy, `degC` for temperature). Send the sample in that unit."
+          description: "Apple's `HKUnit` string for `value`, captured for audit: the server records it and never validates `value`
+            against it. `value` is read in Apple's own `HKUnit` for the identifier — `m` for a length, `kg` for body
+            mass, `kcal` for energy, `degC` for temperature, a `fraction` for the percent-shaped ones (oxygen
+            saturation, body fat) — and HealthLog applies its storage scaling after that. A waist circumference sent as
+            `0.86` in `m` is stored as `86` `cm`; an oxygen saturation sent as `0.98` in `fraction` is stored as `98`
+            `%`. Send the sample in Apple's unit, not in HealthLog's."
         startDate:
           type: string
           format: date-time
diff --git a/src/lib/openapi/routes/measurements.ts b/src/lib/openapi/routes/measurements.ts
index 1f28b049b..f138557cb 100644
--- a/src/lib/openapi/routes/measurements.ts
+++ b/src/lib/openapi/routes/measurements.ts
@@ -58,7 +58,7 @@ const batchEntrySchema = z
       .min(1)
       .max(60)
       .describe(
-        "Apple's `HKUnit` string for `value`, captured for audit. The server does NOT convert from it: `value` is read in the unit HealthLog stores the identifier in (`m` for walking distance, `kg` for body mass, `kcal` for energy, `degC` for temperature). Send the sample in that unit.",
+        "Apple's `HKUnit` string for `value`, captured for audit: the server records it and never validates `value` against it. `value` is read in Apple's own `HKUnit` for the identifier — `m` for a length, `kg` for body mass, `kcal` for energy, `degC` for temperature, a `fraction` for the percent-shaped ones (oxygen saturation, body fat) — and HealthLog applies its storage scaling after that. A waist circumference sent as `0.86` in `m` is stored as `86` `cm`; an oxygen saturation sent as `0.98` in `fraction` is stored as `98` `%`. Send the sample in Apple's unit, not in HealthLog's.",
       ),
     startDate: z.iso.datetime({ offset: true }),
     endDate: z.iso.datetime({ offset: true }),