For any user who comes to a website from a mobile app, there’ll be no referer information. This blog post by Mike Dixon explains it in detail.
In a nutshell, the page referer is a data point that is fetched from a HTTP header on a web page, so any non-web referer wont have any that data set.
So if you are seeing data with the page referrer missing it could be a situation where users have clicked through from links on Twitter, Facebook, etc. within the client, and because the link is on a client rather than a web page, there is no page referer.
A page referer will only be set where a user has landed on one web page having clicked through from another web page.
However the mkt_source and mkt_medium fields should be filled for paid traffic if you are using campaign_attribution.
In these cases you use the mkt_ fields to determine where the traffic comes from. In general you use these as your first port of call for determining where a user comes from, and check the referer data to see if:
- It suggests something contradictory
- Adds some additional useful information. (E.g. tells you that what page URL an ad on a PPC network was served on.)