In StarCraft II, the Terran Siege Tank's artillery transformation is called Siege Mode.
A Siege Tank can switch between its mobile tank configuration and Siege Mode. In Siege Mode, it becomes stationary and gains powerful long-range artillery fire, making it effective against groups of ground units and fortified positions. The trade-off is that it cannot move or attack targets immediately like a mobile tank.
Siege Tanks are one of the most recognizable Terran units because their transformation encourages positional play. Terran players often combine them with infantry, air units, vision sources, and defensive structures to protect their firing line.
The name is sometimes confused with a generic description such as artillery mode. Assault Mode is not the canonical StarCraft II ability name. The older StarCraft games also used the Siege Tank and Siege Mode concept, making it a long-running part of Terran military design.