Answer:
Nothing if the employer doesn't require it.
It is the highest level clearance.
A TS/SCI clearance grants the bearer access to a SCIF, Sensitive Compartmentalized Intelligence Facility. Once a person is briefed into or "SCIF'd" into an intelligence program, they become custodians of HIGHLY SENSITIVE intelligence information that MUST remain in the SCIF. Personal integrity is PARAMOUNT & KEY to acquiring this level of access, and "leaking" intelligence to the outside world carries grave consequences to the individual & national security & is considered treason.