O2 sensor(s) is there an up and a downstream

[GuyS1]

My O2 sensor is putting my check engine light on. I’ve replaced it before. Once myself and then the second time I had a garage replace one I bought from Amazon. It’s possible the second failure was due to a poorer quality sensor (it only lasted 10,000 miles) or it may be due to another problem. I’ve decided to replace it with a more reputable brand from Rock Auto but there seems to be a upstream and a downstream available. My Forester is a 2001. Does anyone know if mine is just an “upstream” or a “downstream” or whether there are actually two sensors?