The Sequence number is a unique 64-bit integer assigned by the Service Bus broker to each message upon acceptance. It functions as the message's unique internal identifier and represents its absolute order within the queue or topic. When implementing message processing logic, the sequence number is the fundamental value used to uniquely identify a message for settlement operations such as completing, abandoning, or dead-lettering. It is also essential for implementing idempotent receivers and for auditing or tracking purposes, making it the most critical value for processing any given message.

A. Session identifier: This is only used for the specific scenario of grouping related messages to ensure they are processed sequentially by a single consumer.

C. Partition key: This is used with partitioned entities to direct related messages to the same partition, primarily for improving throughput and scalability.

D. Time stamp: Timestamps like EnqueuedTimeUtc are metadata for application-level logic (e.g., checking message age) but are not the primary identifier for processing a message.

1. Microsoft Docs: Message sequencing and timestamps. This document explicitly states, "The sequence number is a unique 64-bit integer assigned to a message as it is accepted and stored by the broker and functions as its internal identifier... The sequence number can be trusted as a unique identifier since it is assigned by a central and neutral authority and not by clients."

Source: Microsoft Learn, Azure Service Bus Documentation, "Message sequencing and timestamps", Section: "Sequence number".

2. Microsoft Docs: Service Bus message sessions. This document clarifies the specific use case for sessions: "To realize a FIFO guarantee in Service Bus, use sessions. Message sessions enable joint and ordered handling of unbounded sequences of related messages."

Source: Microsoft Learn, Azure Service Bus Documentation, "Message sessions", Section: "Message sessions".

3. Microsoft Docs: Partitioned messaging entities. This document explains the role of the partition key: "When a client sends a message to a partitioned entity, Service Bus checks for the presence of a partition key. If it finds one, it selects the partition based on that key."

Source: Microsoft Learn, Azure Service Bus Documentation, "Partitioned messaging entities", Section: "Using a partition key".