Jub aap IRCTC me ticket book karne k liye passengers k name fill karte hai tabhi uske side me birth preference ka option rehta hai jisme subhi births rehte hai, aapko jaise SU or SL chahiye to dono passenger k naam fill ker k unka respective birth preference select kare phir payment option pe jaye, agar aapke dwara diya gaya birth preference available hoga to aapko milega.