The Rig is an APC modified to transport building materials to build a temporary port, introduced in Advance Wars: Days of Ruin. It holds all the same transport functions as its predecessor, capable of transporting one Infantry or Mech Unit, and it retains all the supplying capabilities as well.
 What's New
A Rig is capable of building either a Temporary Seaport or Temporary Airport, but only has enough building materials for one such temporary port at a time.
The construction of a Temporary Port depends on the remaining HP of the Rig that is building the Port. Like capturing a City or other Building, building a Temporary Port can be completed in a maximum of two days. The Rig cannot move or get destroyed or else the construction of said temporary Port has to start from scratch.
When completed, the Temporary Port will serve the same purpose that its regular "cousin" will share; the Temporary Seaport will restore HP for Naval Units while the Temporary Airport will restore HP for Air Units. New units cannot be built at these Temporary Ports, however.
Building materials is something that cannot be restored through the same methods that fuel, HP and ammo get restored. Only Admiral Greyfield's CO Power is capable of restoring building materials to a Rig or Carrier.