diff --git a/extension/docs/IntegrationGuide.md b/extension/docs/IntegrationGuide.md
index c3ad4026b..dba48af9c 100644
--- a/extension/docs/IntegrationGuide.md
+++ b/extension/docs/IntegrationGuide.md
@@ -103,6 +103,7 @@ or by asynchronous process like [commercetools-payment-to-order-processor job](h
 There must be at least one CTP payment object of type Adyen
 (`Payment.paymentMethodInfo.paymentInterface = ctp-adyen-integration`)
 AND this CTP payment must have `Payment.custom.fields.merchantReference`.
+The combination of `Payment.custom.fields.merchantReference` and `Payment.paymentMethodInfo.paymentInterface` must be unique across the project. This field must not be changed once set.
 
 ### Validate payment transaction
 Cart's payment counts as successful if there is at least one payment object