Yes there is a horizontal reservation of 5%. However priorities are given in that too. It goes like this

PRIORITY I – Widows/ wards of Defence Personnel/ Para Military Personnel killed in action.

PRIORITY II - Wards of Defence personnel and ex-servicemen/ Para Military Personnel disabled in action and boarded out from service with disability attributed to military service.

PRIORITY III - Widows/ wards of Defence Personnel/ Para Military Personnel who died in peace time with death attributable to Military Service. 

PRIORITY IV - Wards of Defence Personnel / Para Military Personnel disabled in service and boarded out from service with disability attributed to military service. 

PRIORITY V - Wards of serving Defence personnel and ex-servicemen / Para-Military /Police Personnel who are in receipt of Gallantry Awards.

PRIORITY VI - Wards of Ex-serviceman (Defence Personnel only). 

PRIORITY VII - Wards of serving personnel (Defence Personnel only). 

I think you will fall under the VI category so the certificate required will be Original ex-servicemen Identity Card/ discharge book/ PPO (Pension Payment Order).

This wont be applicable for the PwD candidates or the management quota seats.