Azure Service Bus: What is a request and a message?

This is what I found in the docs, but not fully sure if that's what you were looking for:

When creating a queue or subscription client, you can specify a receive mode: Peek-lock or Receive and Delete. The default receive mode is PeekLock. When operating in the default mode, the client sends a request to receive a message from Service Bus. After the client has received the message, it sends a request to complete the message.

When setting the receive mode to ReceiveAndDelete, both steps are combined in a single request. These steps reduce the overall number of operations, and can improve the overall message throughput. This performance gain comes at the risk of losing messages.

Here, under FAQ: https://azure.microsoft.com/en-us/pricing/details/service-bus/

How is the Operations meter calculated for queues and topics?

For brokered entities (queues and topics/subscriptions), an operation is any API interaction with Service Bus service on any protocol.

A send, receive delete for a message that is less than or equal to 64KB in size is considered as one billable operation. If the message is greater than 64KB in size, the number of billable operations is calculated according to the message size in multiples of 64KB. For example, an 8 KB message sent to the Service Bus will be billed as one operation, but a 96 KB message sent to the Service Bus will be billed as two operations. Reading the 8KB message with a lock and then completing or explicitly abandoning the message will be billed as two operations. Renewing the lock on a message also incurs an operation.

Multiple deliveries of the same message (for example, message fan out to multiple subscribers or message retrieval after abandon, deferral, or dead lettering) will be counted as independent operations. For example, in the case of a topic with three subscriptions, a single 64KB message sent and subsequently received will generate four billable operations, one “in” plus three “out”, assuming all messages are delivered to all subscriptions and deleted during the read.

Additionally creating, reading (listing), updating and deleting a queue, topic or subscription will each incur an operation charge.

Operations are API calls made against queue or topic/subscription service endpoints. This includes Management, Send/Receive and Session State Operations.