What makes a meta-unit different from a control group?
Each unit within a meta-unit is always aware of the others.
For example, in the real world, an aircraft carrier could be considered a single unit. But that unit has guns, radar, and various types of aircraft on it. In a sense, it is a meta-unit. In Ashes of the Singularity, players build individual units and then put them together to form a meta-unit based on their strategy of choice.
Units change their behavior depending on the composition of the meta-unit they exist within, and players command each meta-unit as a single entity. This means you don’t have to baby-sit your units in every minor skirmish and instead you can focus on the larger war.