What is "filed against" for?

The purpose of the Filed-Against property is to help automate the assignment of a work item to the team that will handle it.   The idea is that you define a set of "categories" that are concepts that are understood by the submitter of a work item.  Then in the project area, you map each category to a team that is responsible for that category.  That team is then responsible for all work items submitted against that category.   That team is of course free to modify the value of the "filed against" property, to effectively forward that work item to a different team.

When you say "navigate to all the articles filed against a certain category", I assume by "article", you mean "work item"?   If so, you can just create a work item query, asking for all work items that have a particular category as the value of their Filed-Against property.

think of Filed Against as a work queue by team.  you can organize the work by assigning the workitem to different teams of people..

this has NO relationship to ANY structure of the actual technical product being developed or supported with RTC.
(it 'may' but there is no guaranty)

RTC UI's both web AND Eclipse, can and WILL change this value without warning if you drag/drop or move workitems between timelines where the same teams are not in the hierarchy.

Filed against goes to a Category which in turn is associated with the Project Area or Team Area.   The easiest way to find items in a particular Category would be to create query(ies) with one of the criteria being Filed Against.