Empire Reborn Imperial Officers' Manual

... An ISB Major's Rank Insignia ...

An ISB Major (ISB–MAJ) may be slotted into one of two positions:

Aboard smaller ships (less than one wing of pilots) only: the rank of ISB Major is the minimum rank required to act as the shipboard Head of Security.
Aboard large-scale vessels only: the rank of ISB Major is an intermediate rank, to which exceptional Additional Agents may be promoted. These officers may remain Additional Agents onboard the large-scale vessel, or apply for transfer into a vacant Head of Security position onboard a smaller ship.

An ISB Major is granted level 80 X access in #Galactic_Empire.