Topics may be set for Poster events usually by the paper authors if the conference is configured to support poster topics. These topics may be used by a conference for grouping the posters in the poster sessions. Since they are used for physically grouping the poster events in a session only a single topic is supported. You can't have a physical poster in two positions at once. Other events may have their topics set as well but poster events are the most general place for a paper presentation.
The topics are identified and/or confirmed by the program committee. A limited size list is important in order to have a useful grouping of posters.
If you are an author for a poster and the conference has configured topic areas then you can select the topic for your poster. Do this by logging into the website using the email associated with the poster event and then visiting the metadata page for the poster. On the metadata page you can select the topic for your paper from a drop down menu. You can find your poster event(s) for the conference on the My Stuff page.
While you may change your topic at any time, the posters are grouped in a poster session several weeks before the conference, see the Dates information for the conference for this date. Once the groupings are published they will not be recreated so topic changes after this time will no longer be reflected in the session groupings.