What does it take for field in an entity to be able to be used in cotexetual filter, or relationship? I am using a view for an entity.  I had it working until I changed the cardinality of a taxonomy field.  Now that field is no longer available for relationships, or contextual filters.  Every other field defined is available as a contextual filter.

So this makes me wonder what determines whether a field can be used in a contextual filter?
Since you changed the cardinality to more than 1. 

Add a contextual filter of Has Taxonomy Term ID, then scroll all the way down, click on the blue MORE button and checkmark allow multiple values.



Yeah, it's hidden, lol.