Reference segments are segments that you use in other segments. You can define a basic criteria in a segment and then reuse that criteria in other segments.

With reference segments, you can share logic between segments and avoid repetition when creating queries.
By reusing segments you can reduce any errors that might occur while recreating the same logic in multiple segments.
They also provide a way to manage complexity by providing a way to break down query logic into smaller, more digestible and semantic units.
The referencing is dynamic and hence when a segment is updated, all segments that reference it are updated instantly.

Reference segments can be used in both Basic and Advanced segments. For example, you can create a segment for users in the loyalty program and then use it in a segment for loyalty program users residing in a particular area.

Reference segments in basic segments

Reference segments in advanced segments

Nesting of reference segments

When a segment is updated, all segments that reference it are updated instantly. If there are nested segments, the computations are even more complicated. To keep computations efficient, nesting upto only 3 levels breadth and 2 levels depth is accepted.

Breadth of 3 means that a segment cannot have more than 3 reference segments in the segment criteria. For example, If segment A has segments B, C, and D in its criteria, then it has a breadth of 3.

Depth of 2 means that a segment cannot have more than 2 reference segments in the criteria, even if it is not directly referencing these segments. Consider the following examples.