Cocoon proposes a pyramid model for website development, which has four roles:
Management - decide what site should contain, it's behaviour and appearance
Logic - responsible for the interfaces to dynamic content, databases, etc.
Content - write, own, manage site content
Style - responsible for look-and-feel, graphics, etc.
Cocoon tries to explicitly remove the need for a contract between Logic and Style. The others are all linked.
The pyramid structure drives some of the design decisions. For example the Cocoon Processing Instructions, and the W3C XML Stylesheet PI were both discarded because they mix content and style, i.e. the style is referred to from the content even if it's not explicitly styled. These associations are now made through the Sitemap.
Logic is developed in Cocoon using Logicsheets. These are XSP.