'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.