Its every activity if you choose the automatic sync. Obviously it would be selectable if you did it manually, just a bit more work involved. Any reason for not wanting every activity synced? You can always remove an activity from strava once synced which is far easier than manually adding activities.
Former Member you can't, but what some people do, including me, is to set all Strava activities to private by default. Then you just set the activities you want to share to public. It's just one extra click/tap, assuming that you already edit your Strava activities' name and description.
