Every page or module is a Content entity associated with a specific ContentType. ContentTypes are unique content like an Article, Profile, Form, Video, etc. The Content is a simple generic record that exists to provide a permanent id for each page. This permanent ID is necessary because a content (i.e. the page or content) can be versioned and each version will have a different ID, so there has to be one unifying ID that all other associated entities can reference.

Content Associations

Content (content)

The Content entity is a versionable entity, so Content which has a lot of associations with other entities that provide additional information about the page, but these are the most important associations to understand how a page is a built and how it interacts with the site

ContentType (content.contentType)

The ContentType entity defines the type of content of each Content, e.g. Article, Form, Map, Video, etc. These Content Types are defined in a master Content Type list, and are available based on the services that a site is subscribed to. The Content Types point to the Controller and Twig Templates that render the specific page. The ContentType also determines the correct Meta entity to associate with the version.

Routing (content.routing)

The Routing entity defines Friendly URLs that point to a specific Content (page). One or more Routing entities can be associated with a Content. There will always be one primary route and additional routes will be aliases that redirect to the primary route.

ContentVersion (content.version)

The ContentVersion entity is the versionable part of the Content entity, whichthat contains the majority of the information about a Content which can change, e.g. Title, Author, Date, Content, Images, etc. Content will automatically version when new users edit them or after a specific period of time (or when manually requested), and a new ContentVersion is cloned from the current ContentVersion. This keeps a revision history for every page. When previewing a site, the last modified version is used, but when viewing the live site, the current Published version is used.

The ContentVersion contains the standard fields that most pages or modules will need, e.g. Title, Author, Date, main Content, main Image, etc. This is done so that it is easier to reference the content of each entity in a list without having to attach a lot of other associated entities. These ContentVersion fields should be used by the Content Type whenever possible, i.e. some Content Types like “Article” may not even have any custom fields in the associate Article (Content Type) entity. But many Content Types do need unique fields, so all ContentVersions reference a related Content Type entity.

ContentVersion Associations

The Content can be associated with one or more tags that are themselves associtaed with one or more Streams, (list pages that display all the content associated with the tags).
A content can specify a unique template shell design to use (the look of the container around the content), and this preference is associated with the ContentVersion so that it can be previewed before the ContentVersion is published.
A content can specify a unique layout design to use (the look of the content area), and this preference is associated with the ContentVersion so that it can be previewed before the ContentVersion is published.

Meta (content.version.meta)

The Meta entity is a dynamic attachment point for different ContentType entities where unique data is stored for this type of content (e.g. Article will have slightly different data storage needs than Profile so we the meta lests us keep the ContentVersion entity focused on just the most commonly stored data relevant to all Content). Content.ContentType specifies which kind of Meta entity should be joined and then our code finds finds the correct ContentType entity (e.g. Article) and joins it to the ContentVersion. Therefore the meta entity iterates a new record everytime the ContentVersion changes.