It is the reactance of cable system e.g. when three single core cables are laid on cable tray etc.
Equivalent star reactance depends on how cables are physically installed. The reactance is different if cables are installed flat or in star shape. The cable manufacturer usually indicates in data-sheet for what kind of installation shape the equivalent star reactance is given.
I'am not absolutely sure about this but please write 'equivalent star reactance' in 'Google'-search engine and you will find lots of answers.