In that case it obviously means a cymbal or some other ride voice. Otherwise it means whatever the key for that particular page says it means. I use it only when I want to include an optional note in a page of exercises-- I'm not aware of anyone else doing that.
Do check out PAS and Berklee publications, they've made some efforts at standardizing drum notation.
Odd..never seen that one before. The solid note on that line can represent a high tom.. and the circle around the note is sometimes used to show a bell of a ride cymbal - but with an 'x' inside.
I've seen the circle around an 'x' to indicate an open hihat.. but also as a small 'o' above the note stem.
I've also seen the circle around a solid note to indicate cross-stick.
So.. it's a pretty mixed up message.. maybe the composition software the author used is at fault.
But if I had to guess.. judging by the position on the staff and general pattern.. it's either the bell of a ride or an open high hat - or the bell of a high tom - whatever that is! Lol