Reusable Block composition belongs to the Source and remains intact in the normal document, Focus mode, and Slides mode.

Use layout: center for a deliberately centered Block.

Use layout: wide for tables, diagrams, interfaces, and other content that needs more room than the normal prose measure.

Logical start and end names adapt without changing the Markdown content order.

The same layout metadata works outside Player and survives mode changes.

Image roles and Block layouts compose without becoming Player directives.

Cover, section, quote, fact, full, and end describe presentation treatment—not portable Block composition.