diff --git a/src/lib/pdf/format.test.ts b/src/lib/pdf/format.test.ts
index e54126c..cd667b1 100644
--- a/src/lib/pdf/format.test.ts
+++ b/src/lib/pdf/format.test.ts
@@ -110,8 +110,6 @@ describe("effectiveQuoteDate", () => {
   });
 });
 
-
-
 describe("effectiveValidityDays", () => {
   it("uses quote.validity_days when set to a number", () => {
     expect(effectiveValidityDays({ validity_days: 60 })).toBe(60);
diff --git a/src/lib/pdf/format.ts b/src/lib/pdf/format.ts
index 150783a..75ae708 100644
--- a/src/lib/pdf/format.ts
+++ b/src/lib/pdf/format.ts
@@ -40,7 +40,9 @@ export function quoteSubtotal(items: { quantity: number; selling_price: number }
 }
 
 /**
- * Sum of (order_qty ?? quantity) * cost_price across PO items. Differs
+ * Sum of effective-qty × cost_price across PO items, where
+ * `effective-qty` is `order_qty` when set (non-zero) and falls back to
+ * `quantity` otherwise. Differs from `quoteSubtotal()` in two ways:
  * from `quoteSubtotal()` in two ways:
  *   1. Multiplies by `cost_price` (the supplier-facing price), not
  *      `selling_price` (the customer-facing price).