Transport modes is used to determine which method to be used to mount the snapshot in proxy.

• Transport Mode SAN (storage area network) - SAN mode is supported for directly connected storage. The VSA should have access to the datastore LUNs that provide storage for VM disks. Data is read directly from the storage where VM resides. WITHOUT going through the ESX host or transferring data over the local area network (LAN). The ESX host is contacted only to coordinate access to the LUN.
• HotAdd In HotAdd mode, the Virtual Server Agent is installed on a VM residing on an ESX Server. In HotAdd mode, the data volumes CONTAINING the VMs to be backed up are AUTOMATICALLY mounted to the proxy, so they can be accessed by the proxy as a local DISK. 
• Local Area Network (NBD and NBDSSL) NBD (network block device) & NBDSSL (encrypted NBD) transmit data over the TCP/IP connection between the ESX server and the proxy computer.

NBDSSL is similar to NBD mode, but data transfer between the proxy computer and the ESX server is encrypted.

Transport modes is used to determine which method to be used to mount the snapshot in proxy.

NBDSSL is similar to NBD mode, but data transfer between the proxy computer and the ESX server is encrypted.