What is the meaning of the field:delta content type offered in contextual filters in views? When creating a new contextual filter for a view the list of types includes options such as content:spec (field_spec:delta) where spec is the name of a field I have attached to the content type accepted for the view.  It appears on some, but not all of the fields offered in the list.  I cannot find a reference on this field. What is this (field_name:delta)?
It appears only for multi-value fields. It refers to the order of the values. A delta of 0 is usually the first item, 1 is the second, etc. This can be used to restrict results to ones that have values for a specific delta.