The number of command centers deployed in combat zones drove the development of standard schematics to upgrade them into a dedicated military role. One variant was the planetary fortress. The Ares planetary fortress entered service following the Brood War and featured twin Ibiks cannon and heavier armor.
We can now outfit the command center with the twin Ibiks cannon and additional structure plating. Please note that this upgrade, nicknamed the planetary fortress, must be triggered after the command center has been built. Also note that a command center that has been upgraded to a planetary fortress can no longer lift off because the added weight is too much for the command center's atlas boosters.
We now have the ability to train two SCVs simultaneously, allowing our early mineral and vespene production to ramp up twice as quickly. This breakthrough, made possibly by recent insights into protoss containment fields, should help us get a stronger foothold early in future missions.
The planetary fortress used to be an independent structure enabled by the factory. In regards to command center variants, most work went into the fortress, with the results effectively being ported to the orbital command, which was designed later. It used to have lift off properties, but it was ultimately decided to keep it as a grounded structure.
The planetary fortress originally had this upgrade, researched at an engineering bay:
Planetary Fortress Range
Increases the range of the planetary fortress' attack by +1.