Communication between Configuration Manager sites must happen through one or more senders, and these senders can only be installed or deployed on primary or secondary site server systems. Instead of providing connectivity to sites, senders use EXISTING network connections for MANAGING connectivity, ENSURING the integrity of transferred data, recovering from errors, and closing connections when they are no longer required. SCCM has the following types of senders: 

• Standard Sender: By default, all primary and secondary sites are configured with the standard sender. In the case of site-to-site communications over a LAN (Local Area Network) that uses a supported protocol, you do not have to install an additional sender. 
• Courier Sender: Primary and secondary sites are all configured with the courier sender by default. However, it cannot be created or DISPLAYED in the Configuration Manager console. The reason for this is that you need to start it manually from the Configuration Manager programs folder on your Start menu. As opposed to SENDING data over the network, courier senders are essentially used for software distribution in order to send package data via physical media to another site.