Sigma group was a common label used for Delta-class DX-9 stormtrooper transports. These transports were generally stationed on Imperial warships or space stations.
These vehicles were often used to disable and then board enemy ships, for example, the BFF-1 bulk freighter known as Onece, or to perform heavy rocket attacks, like the one carried out against the Rebel cruiser Lulsla.
Omega was also another typical designation for the Empire's DX-9 transports.