Substack Segment Campaigns

Turn a subscriber export into segments the user builds in Substack, plus copy that reaches only that segment.

Two facts that decide everything

1. Notes are public. A Note can't reach a segment, and anything written "to lapsed readers" in a Note tells the whole feed they left. Never write a Note for a segment campaign.
2. The export is private data about people who never agreed to be discussed. Follow references/untrusted-content.md section 2.

What you output

1. The segment, in Substack's filter terms, so the user builds it in the dashboard:

Segment: Lapsed paid
Filters: Subscription type = paid · Status = cancelled
Optional: Last opened within 60 days (still reading)
Size in this export: 12 (4 opened in the last 30 days)