You can narrow down the topics that are returned by passing parameters to the request that you use to retrieve the feed.
How to discover the URL depends on which type of forum you are working with.
Value of the href attribute of the <link> element with the rel="http://www.ibm.com/xmlns/prod/sn/forum-topics" attribute value for each community entry returned by one of the communities feeds.
Value of the href attribute of the <link> element with the rel="replies" attribute value for each forum entry returned by one of the forums feeds.
You can use one or more of the input parameters to limit the number of entries returned in the feed. Separate multiple parameters with an ampersand (&).