I have been playing around using the Search Refinements web part to do some interesting things but one thing that I did come across was the MetadataThreshold on each property.
Now what this value does is actually control the appearance of the refiner when a query is pumped in. When a result set comes in the refinement panel will count the metadata that matches the particular property and compare it against the MetadataThreshold. If it exceeds is then it will show the refiner, otherwise it won't appear.
Now this is very important because I have seen countless forum posts about why the filter is not appearing so lets show a demo. In this Example my Managed Metadata Columns refiner is set to 1 (and counts turned on see another post for this), therefore if the set of results matches exceeds more than 1 match for all the possible options for the metadata set we will get our refiner shown:
Now if I go into the Refinement Panel web part and edit the Filter Definition Category changing the value to '5' which is one more than our current matches what will happen?
Thats right it wont appear! It doesn't mean that there was no results in the set that matched that peice of metadata, just that there wasn't enough to be higher than the MetadataThreshold and hence the refiner will not be shown.
So what does this mean? Well basically you do have control over which refiners are shown but be careful. Setting every possible refiner to 1 means that the refiner panel might get crowded (also you have to set the 'Number of Categories to Display' to more than 6 so you can see them all) but setting the limit too high means that it might not be shown for small results sets.
Need more information about the refinement panel, try these links: