The Strategic Command is an organizational unit. It is used to handle everything in between the Supreme Command and the assault teams.
The strategic commands are not visible on the campaign map, only assault teams and reserves can be seen there.
The Strategic Commands makes the overall strategic decisions, handles the logistics of getting men and tanks to the frontlines, setting up defences in areas that are under threat and the management of war factories.
There is no army resources directly attached to a strategic command and therefore it cannot attack or defend (a later upgrade is planned to include the HQ of the strategic command to enable special missions like assassinations and infiltration). Army resources owned by a strategic command is placed in reserve units.