I think it differs from conference to conference. I submitted an abstract to one that was as you described, and the instructions asked us to specify which symposium we felt was the best fit, although all abstracts went to a central place. In the end, I think the conference created an additional symposium, because mine and similar others ended up in a group that I don't think was on the original list! Are there any more details on the application form, e.g. a space for you to put the symposium name?