diff --git a/mountpoint-s3/src/upload.rs b/mountpoint-s3/src/upload.rs
index ce93f08f0..213d1f677 100644
--- a/mountpoint-s3/src/upload.rs
+++ b/mountpoint-s3/src/upload.rs
@@ -100,7 +100,10 @@ pub struct AppendUploader<Client: ObjectClient> {
     mem_limiter: Arc<MemoryLimiter<Client>>,
     buffer_size: usize,
     server_side_encryption: ServerSideEncryption,
-    checksum_algorithm: Option<ChecksumAlgorithm>,
+    /// Default checksum algorithm, if any, to be used for new S3 objects created by an [AppendUploader].
+    ///
+    /// For existing objects, Mountpoint will instead append using the existing checksum algorithm on the object.
+    default_checksum_algorithm: Option<ChecksumAlgorithm>,
 }
 
 #[derive(Debug, Error)]
@@ -134,7 +137,7 @@ where
         mem_limiter: Arc<MemoryLimiter<Client>>,
         buffer_size: usize,
         server_side_encryption: ServerSideEncryption,
-        checksum_algorithm: Option<ChecksumAlgorithm>,
+        default_checksum_algorithm: Option<ChecksumAlgorithm>,
     ) -> Self {
         Self {
             client,
@@ -142,7 +145,7 @@ where
             mem_limiter,
             buffer_size,
             server_side_encryption,
-            checksum_algorithm,
+            default_checksum_algorithm,
         }
     }
 
@@ -160,7 +163,7 @@ where
             initial_offset,
             initial_etag,
             server_side_encryption: self.server_side_encryption.clone(),
-            checksum_algorithm: self.checksum_algorithm.clone(),
+            default_checksum_algorithm: self.default_checksum_algorithm.clone(),
             capacity: MAX_BYTES_IN_QUEUE / self.buffer_size,
         };
         AppendUploadRequest::new(
diff --git a/mountpoint-s3/src/upload/incremental.rs b/mountpoint-s3/src/upload/incremental.rs
index 632b9beee..20d270ca0 100644
--- a/mountpoint-s3/src/upload/incremental.rs
+++ b/mountpoint-s3/src/upload/incremental.rs
@@ -20,6 +20,12 @@ use crate::ServerSideEncryption;
 use super::hasher::ChecksumHasher;
 use super::{AppendUploadError, BoxRuntime, ChecksumHasherError};
 
+/// Handle for appending data to an S3 object.
+///
+/// This request contains a buffer that can be written to,
+/// before being pushed to the [AppendUploadQueue]
+/// which may queue some number of buffers while appending data to the S3 object
+/// over multiple subsequent S3 PutObject with offset requests.
 #[derive(Debug)]
 pub struct AppendUploadRequest<Client: ObjectClient> {
     /// The current buffer, initialized lazily on write.
@@ -107,19 +113,35 @@ where
     }
 }
 
+/// Output from the [AppendUploadQueue],
+/// providing information about the last PutObject request
+/// or providing the required checksum algorithm when first initialized.
 #[derive(Debug)]
 enum Output {
+    /// Initial output state, dictating which checksum algorithm should be used for all append requests.
     ChecksumAlgorithm(Option<ChecksumAlgorithm>),
+    /// Result of a PutObject with offset request.
     Result(PutObjectResult),
 }
 
+/// Queue for an active 'append' to an S3 object.
+///
+/// This struct has message channels whose queues may contain a number of buffered parts to be appended to an object.
+/// The struct is responsible for taking requests from the queue and initiating the S3 PutObject with offset requests.
+///
+/// Requests should be sent to this struct using [AppendUploadRequest::write].
 #[derive(Debug)]
 struct AppendUploadQueue<Client: ObjectClient> {
+    /// Channel handle for sending buffers to be appended to the object.
     request_sender: Sender<UploadBuffer<Client>>,
+    /// Channel handle for receiving the result of S3 requests via [Output] messages.
     output_receiver: Receiver<Result<Output, AppendUploadError<Client::ClientError>>>,
     mem_limiter: Arc<MemoryLimiter<Client>>,
     _task_handle: RemoteHandle<()>,
     /// Algorithm used to compute checksums. Initialized asynchronously in [get_buffer].
+    ///
+    /// Outer [Option] represents if the algorithm configuration is known yet,
+    /// while the inner [Option] having value [None] indicates that no algorithm should be used.
     checksum_algorithm: Option<Option<ChecksumAlgorithm>>,
     /// Stores the last successful result to return in [join].
     last_known_result: Option<PutObjectResult>,
@@ -133,7 +155,10 @@ pub struct AppendUploadQueueParams {
     pub initial_offset: u64,
     pub initial_etag: Option<ETag>,
     pub server_side_encryption: ServerSideEncryption,
-    pub checksum_algorithm: Option<ChecksumAlgorithm>,
+    /// Preferred checksum algorithm for new objects.
+    ///
+    /// If the object already exists, its current algorithm will be used instead.
+    pub default_checksum_algorithm: Option<ChecksumAlgorithm>,
     pub capacity: usize,
 }
 
@@ -157,6 +182,10 @@ where
         let task_handle = runtime
             .spawn_with_handle(
                 async move {
+                    /// Send result of S3 requests to the [AppendUploadQueue].
+                    ///
+                    /// Returns `true` if output was sent successfully.
+                    /// When the output cannot be sent, buffer receiver will be shut down.
                     async fn send_output<Client: ObjectClient>(
                         sender: &Sender<Result<Output, AppendUploadError<Client::ClientError>>>,
                         receiver: &Receiver<UploadBuffer<Client>>,
@@ -186,7 +215,7 @@ where
 
                     let first_output = if offset == 0 {
                         // If we are creating a new object or overwriting (truncate), use the default checksum algorithm.
-                        Ok(Output::ChecksumAlgorithm(params.checksum_algorithm))
+                        Ok(Output::ChecksumAlgorithm(params.default_checksum_algorithm))
                     } else {
                         // We are appending to an existing object, find out which checksum algorithm it uses.
                         match client
@@ -217,6 +246,7 @@ where
                         return;
                     }
 
+                    // Main loop waiting on new buffers to append to the S3 object.
                     while let Ok(buffer) = request_receiver.recv().await {
                         let buffer_len = buffer.len();
                         let result = append(&client, &bucket, &key, buffer, offset, etag.take(), sse.clone())
@@ -320,6 +350,9 @@ where
         )?)
     }
 
+    /// Wait on output, updating the state of the [AppendUploadQueue] when next output arrives.
+    ///
+    /// Returns `true` when next output is successfully consumed, or `false` when no more output is available.
     async fn consume_next_output(&mut self) -> Result<bool, AppendUploadError<Client::ClientError>> {
         let Ok(output) = self.output_receiver.recv().await else {
             return Ok(false);