The TOC is a new area added as part of The Underground expansion and forms the central hub for the new missions set from the dlc.
Accessed from The Terminal (down the steps) is is open to players with the expansion, onc they have completed the initial set up mission that leads to its use. In the TOC you will find the Underground Operations desk where you can configure and activate the missions (in various stages and difficulties) and matchmake with others to complete the missions. Finding a group is necessary on the harder difficulties.
The TOC also has its own Vendor. The Underground Vendor, who stocks Weapons, gear and mods similar to the special weapons vendor in the Base of Operations. You can buy high end and gearset items here for Phoenix Credits, but you need to first reach certain Underground ranks to be eligible.