Until now search folder rules were "additive" or "removing". This mean when only one of the "additive" and none of the "removing" match rules did match an item it was displayed by the search folder. User feedback over time did show that this is not always intuitive and does not match each use case.
To improve this the search folder properties for 1.5 have changed:
Instead of the long logic explanation there are now two radio buttons allowing to define the intended logic. With "Any Rule Matches" you can create search folders that for example match several rare terms. And using "All Rules Must Match" you can filter all feeds for items on a specific topic identified by one or more keywords.
To give proper credit I must mention that this change was motivated by the searching dialog of RSSOwl (a great platform independant Java based aggregator) which has even more nice feature like instant preview and live updating.