diff --git a/lib/core/src/model.rs b/lib/core/src/model.rs
index 54cfaed0..258950bb 100644
--- a/lib/core/src/model.rs
+++ b/lib/core/src/model.rs
@@ -349,7 +349,7 @@ pub struct PrepareSendRequest {
     pub destination: String,
 
     /// Should only be set when paying directly onchain or to a BIP21 URI
-    /// where no amount is specified
+    /// where no amount is specified, or when the caller wishes to drain
     pub amount: Option<PayOnchainAmount>,
 }
 
diff --git a/lib/core/src/sdk.rs b/lib/core/src/sdk.rs
index 3a5ba014..9027c2b5 100644
--- a/lib/core/src/sdk.rs
+++ b/lib/core/src/sdk.rs
@@ -662,7 +662,7 @@ impl LiquidSdk {
         }
     }
 
-    /// Estimate the lockup tx fee
+    /// Estimate the lockup tx fee for Send and Chain Send swaps
     async fn estimate_lockup_tx_fee(
         &self,
         user_lockup_amount_sat: u64,
diff --git a/packages/dart/lib/src/model.dart b/packages/dart/lib/src/model.dart
index b7a32ee6..9b171aba 100644
--- a/packages/dart/lib/src/model.dart
+++ b/packages/dart/lib/src/model.dart
@@ -1005,7 +1005,7 @@ class PrepareSendRequest {
   final String destination;
 
   /// Should only be set when paying directly onchain or to a BIP21 URI
-  /// where no amount is specified
+  /// where no amount is specified, or when the caller wishes to drain
   final PayOnchainAmount? amount;
 
   const PrepareSendRequest({