'General availability' and 'tatkal availability' are the two  available options. General availability is actually the normal availability  status. For those who plan their journey at the last moment and cannot secure  reserved accommodation, Railway provided 'Tatkal' booking facility for them on  payment of premium charges on a 'first-come-first-served' basis. If your date  of journey is in the next two days and the general availability status is 'wait  list', you may only see the tatkal option.