The idea is simple: apply a boost to a document if it matches a tag, where matching a tag means that given a field name and a value thats part of the scoring function definition, we apply the boost if the field value equals the given value or for multi-valued fields if any of the values equals the given value.
This allows people to tag products as promotion and get a generic promo boost or stuff like that.
If we had conditions on all scoring functions, this could just be constant scoring with a condition. The scenario is so clear cut that I wonder if it would be better to have an explicit case for it though.
Tag boosting is now available – see http://azure.microsoft.com/blog/2015/02/05/personalizing-search-results-announcing-tag-boosting-in-azure-search/.
Azure Search team