When composing a filter it's required that the attributes being defined need to be written with the correct name and case, otherwise they will fail. The name and case are based on how they are defined in the domsrvr dictionary (review of the site\ddict.sch displays all attributes). It's important to note that most knowledge attributes (which reside in the "SKELETONS" table) are written in all upper case.
For example if you want to filter based on all knowledge documents that have a Title that contains the word "Install" the "Additional Filter" should be written as:
TITLE like 'Install%'
Another example might be if you wanted to only export knowledge documents that were published, you can use a statement like:
STATUS_ID = 70