I am aware of several posts regarding the benefit of the second list of caches under the 'challenges' tab - and the various discussions around how it is comprised etc.
There is a comment at the bottom of the second column of caches saying "All noted challenges which aren't logged or owned are listed. For exclusion they can be tagged in the logs with [NOT-SIGNED].
This does not appear to work - I have a very long list there which comprises of various challenges caches that I have either signed (not qualified) or qualified (not signed) but logs added. I felt I did not need all of the caches in that list so have tried to comply with the above by adding [NOT-SIGNED] to the logs in the hope that they would disappear from the list - they haven't (I did this several weeks ago)