Reference and active migration guide for Sentry's cell architecture. Explains what cells and localities are and why they're different, how requests reach cells via Synapse API routing, ingestion routing, and the control silo gateway, and how to safely query cross-cell data without silently missing results. The migration section covers how to do migration work: draining the URL_NAME_TO_ACTION registry in test_urls.py to zero (with a recipe for each action type), rolling deploy safety and the two-phase pattern required by independent sentry/getsentry deploys, and the region -> cell rename including what not to rename (DB columns, AWS refs, uptime regions, billing address). Also documents known issues with proposed fixes: org listing and creation without a slug, integration TeamLinkageView routing, Jira cross-cell fan-out, and relocation endpoint routing.