As an example I might have available two encodings of an episode. Both are 720p HDTV, but while one is a scene release, the other is an internal release by the tracker or its users. In this case they tend to be tagged, and so my regex could match \binternal\b on one of them and so prefer that over the other.
Or there might be multiple 3D releases and I don't know what will be available but have a preference for full SBS. If that doesn't exist I'd still take half, or under over. So in this case I'd add a rule to say I prefer one against otherwise equal files when it matches this SBS regex.
Or there might be two groups uploading the same episodes and I tend to find group A has better quality, so will pick a group A release when available.