Compact layouts for mobile record highlights
Key Takeaway
Creating a custom compact layout doesn't turn it on. The Salesforce mobile app keeps using the system default compact layout until you set your layout as the primary compact layout in Compact Layout Assignment.
A compact layout is the short list of key fields that appears at the top of a record in the Salesforce mobile app. When a user opens a record, the record's header shows these fields as highlights in the record highlights area. Admins customize the compact layout so users see the most important details about a record at a glance, without scrolling.
Where a compact layout is used
The same compact layout also drives record headers in Lightning Experience and in the Outlook and Gmail integrations. There is no separate mobile-only compact layout, so a change to a compact layout shows up in both Lightning Experience and the Salesforce mobile app.