Templates, Policies, and Editable Templates
Editable templates separate structure from content. A template (under /conf/<site>/settings/wcm/templates) is composed of a structure, initial content, and policies. The structure defines the layout and locked components; initial content seeds new pages; policies (stored under /conf/.../settings/wcm/policies) control component behavior and are assigned through the template's policy mappings. Static templates (under /apps) are the legacy approach and cannot be edited by authors.
Understand the relationship: template type defines allowed templates, template defines structure and policy mappings, policies define design properties (e.g., allowed components, responsive settings). The Template Editor is where authors/developers build these; the Layout Container and responsive grid underpin page structure.
Traps: confusing structure vs initial content (structure is inherited and locked, initial content is copied and editable); assuming policy changes require code deploys (they don't-they're content); mixing up design dialog vs edit dialog. Another trap: a component appears in the editor but not on the page because the policy's allowed components don't include it.
Worked example: to restrict a container to only allow the Teaser component, edit the template's policy for that container and add Teaser to allowed components, then verify the policy mapping points to the correct policy.
Rule of thumb: if an author can't drop a component, check the policy's allowed components before checking the component itself.