In many basilicas and shrines there are side altars where one can light a candle for a special request. Saints have particular areas that are conferred in relation to the life or death of that saint.
The main altar is always reserved for Mass which is only celebrated at that altar. In the case of the main or Sacred Altar, it is where sacrifice is offered.
Altar in the case of side altars is derived from the Latin word ex-altus which means to exalt, lift up in praise.
There is never any question in a church which is the main altar where Mass is celebrated, and which are side altars dedicated to saints and, of course, Mary.
Christ was sacrificed once far all and is now sitting at the right hand of the Father.
He is NOT continually being sacrificed currently.