Transport classes are service libraries for drivers in the SCSI lower layer, which expose transport attributes in sysfs.
The file drivers/scsi/scsi_transport_fc.c defines transport attributes for Fibre Channel.
The file drivers/scsi/scsi_transport_iscsi.c defines transport attributes for the iSCSI class, which sends SCSI packets over TCP/IP connections.
The file drivers/scsi/scsi_transport_sas.c defines transport attributes for Serial Attached SCSI, a variant of SATA aimed at large high-end systems.
The SAS transport class contains common code to deal with SAS HBAs, an aproximated representation of SAS topologies in the driver model, and various sysfs attributes to expose these topologies and management interfaces to userspace.
In addition to the basic SCSI core objects this transport class introduces two additional intermediate objects: The SAS PHY as represented by struct sas_phy defines an "outgoing" PHY on a SAS HBA or Expander, and the SAS remote PHY represented by struct sas_rphy defines an "incoming" PHY on a SAS Expander or end device. Note that this is purely a software concept, the underlying hardware for a PHY and a remote PHY is the exactly the same.
There is no concept of a SAS port in this code, users can see what PHYs form a wide port based on the port_identifier attribute, which is the same for all PHYs in a port.
The SATA transport is handled by libata, which has its own book of documentation in this directory.
The file drivers/scsi/scsi_transport_spi.c defines transport attributes for traditional (fast/wide/ultra) SCSI busses.
The file drivers/scsi/scsi_transport_srp.c defines transport attributes for SCSI over Remote Direct Memory Access.