SharePoint 2013 Search: Full-Text Index and Context Weight Groups - What is wrong with my rank model?

In a previous blog, I talked about the important relationship between relevancy, rank models, managed properties, the full-text index and context weight groups.  If you are going to doing any relevancy tuning, you must understand how managed properties map to context weight groups in the full-text index and how, through the rank model, you can assign weights to context weight groups by referencing a managed property belonging to a group.  

Here is a TechNet article which gives a simple overview of that relationship.

What the article leaves out is that some contexts, in addition to Context 0, do not contribute to Term Frequency scoring. For example, Context 3 (ClickQueryTerms), while used for recall, will not receive term frequency scores for hits in that context.  If you added managed properties to Context 3, terms in those managed properties will not contribute to a relevancy score.  In the screenshot from the ExplainRank.aspx page below, you see that despite the query term 'update' being present in Context 3 (TF=1) the Weighted TF and Term Score is 0.

If want to add a managed property to contribute to a relevancy score, assigned it to a Context that has similar data in importance and length. Stick to Context 1 (Title), Context 4 (Keywords), Context 5 (Author), Context 7 (Body) or use "unassigned" contexts like 9, 10, 12, 13.  I've tested these contexts and relevancy works correctly.  You might have success with other contexts, but you need to thoroughly test it.