The date [day,week,month,year] x1=d1=k1 is recurrence/repetition of the date [day,week,month,year] of the first event x2=d2, for the x3=k3'rd time, in system x3.
Named reference. It converts a sumti into another sumti. The converted sumti points to the referent the name of which is the referent of the unconverted sumti.