What criteria cannot be used to create a Custom Segment?

A segment is a subset of your Analytics data. For example, of your entire set of users, one segment might be users from a particular country or city. Analytics includes predefined segments (System Segments) that you can use as provided, or that you can copy and edit to create new custom segments. You can also build your own segments from scratch. In addition, you can import segments from the Analytics Solutions Gallery, a free marketplace where Analytics users share segments and other solutions they’ve developed. Ad Type is not a criteria to create custom segments. Tip: You can have up to 4 segments applied to your reports at any one time.