If not a secret it'll do until a real secret comes along. This is from Boeing's website:
E-6 TACAMO (Take Charge and Move Out) airplanes support the U.S. Navy's ballistic missile submarine force. They provide survivable, reliable, and endurable airborne command, control, and communications between the National Command Authority (NCA) and U.S. strategic and non-strategic forces.
Functions and Capabilities
The E-6 TACAMO consists of the E-6A, a modified Boeing 707-320B with CFM-56 engines, and the E-6B, a modified E-6A equipped with an airborne launch control system (ALCS) capable of launching U.S. land based intercontinental ballistic missiles.
The fleet provides:
- a faster and longer-range aircraft than the older propeller-driven EC-130 and EC-135.
- continuous airborne alert coverage for both the Atlantic and Pacific oceans.
- a very-low-frequency (VLF) dual trailing wire antenna system to permit one-way, emergency communications to submerged submarines.
- operation from short airfields in emergencies.
Apparently the plane is designed for a very specific mission...