What criteria cannot be used to create a Custom Segment?
- Ad type
- Sequences of user actions
The segment builder lets you create the component filters of a segment based on Analytics dimensions and metrics. You choose a dimension or metric, a comparative operator, and you enter a value to set the condition for the filter.
Conditions and Sequences filters are also based on dimensions and metrics. they offer additional options. Sequences filters let you determine whether the sequence begins with the first user interaction or with any user interaction.
Read more here: https://support.google.com/analytics/answer/3124493