Marking as known is a way of telling the crowd not to report this bug again. Reason for this: you're aware of it, but it doesn’t meet the threshold or specifications of whatever you’re looking for. It's still necessary to monitor the volume of the known bug list to leverage the out of scope section or Feature descriptions, as this list will continue to grow.
Reject a bug when it falls outside of the specified testing guidelines or is not, in fact, an actual bug.
Mark a bug as known when you want to tell testers not to report this bug again. (Why? You may be aware of the bug, but it doesn’t meet the threshold or specifications of what you’re looking for.)
Note: You shouldn't reject a bug if it's on a device not relevant to you; instead, you should update your device preferences.