The terms "na", "unknown", "not set", and "(not set)" displayed in analytics reports all mean the value for a given field of a transaction is not available. Please reference http://apigee.com/docs/analytics-services/content/why-am-i-seeing-analytics-entity-named-na regarding the detailed explanation for possible reasons.
Starting with the Edge Cloud Release 150318, deployment complete on April 2nd, customers would only see "(not set)" representing the missing information for transactions captured after the release. This also means the data captured prior to the release would and could have different values representing the same meaning in the analytics reports.
Link to documentation related to this http://apigee.com/docs/analytics-services/content/why-am-i-seeing-analytics-entity-named-na
The link to "http://apigee.com/docs/analytics-services/content/why-am-i-seeing-analytics-entity-named-na" is broken.. "Page not found"
mentioning @docs to bring attention to the docs team
There was a broken URL redirect. All fixed now. Thanks for raising this.