Append Blob is specifically optimized for operations where we need to keep adding data to a blob in chunks without modifying the already existing content. Every newly added data appends to the end of the blob, operation being referred as appending.

Append Blob is optimized for fast append operations, making it ideal for scenarios where the data must be added to an existing blob without modifying the existing contents of that blob (Eg. logging, auditing).

Unlike other blob types, append blob needs to be created as empty and then contents to be appended into the blob.When you modify an append blob, blocks are added to the end of the blob only, via the Append Block operation. Updating or deleting of existing blocks is not supported. Unlike a block blob, an append blob does not expose its block IDs.

Each block in an append blob can be a different size, up to a maximum of 4 MiB, and an append blob can include up to 50,000 blocks. The maximum size of an append blob is therefore slightly more than 195 GiB (4 MiB X 50,000 blocks).

https://medium.com/lets-azure/creating-and-managing-append-blob-with-azure-storage-rest-api-abb8752aa85f