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+# Content references
+
+Content references are a concept in the GitBook API for linking between spaces, pages, anchors, files, and other entities. A definition of a content reference can be found in the OpenAPI spec under `api.ContentRef`.
+
+Every link in GitBook - even to external URLs - is defined as a content reference.
+
+Content references can be, by design, relative. We call "resolving a content ref" the process of taking a potentially relative ref along with the current context of the user and producing an absolute link:
+
+```typescript
+async function resolveContentRef(contentRef: api.ContentRef, context: GitBookSpaceContext): Promise<ResolvedContentRef>;
+```
+
+Relative content references are resolved against the context:
+
+```typescript
+const ref = { kind: 'page', page: 'my-page-id' }; // a relative ref to a page in the current space
+const resolved = await resolveContentRef(ref, { space: 'my-space-id' });
+// resolved.url = ../../my-page-id
+
+const resolved = await resolveContentRef(ref, { space: 'another-space-id' });
+// resolved == null, because the page does not exist here
+```
+
+## Reusable Content
+
+Reusable content presents an interesting challenge for content references:
+
+- Reusable content belongs to a parent space.
+- All content refs defined in the reusable content will be relative to the parent space. For example, for a content ref inside `my-space`, `{ kind: page, page: 'my-page-id' }` refers to the page `my-page-id` inside `my-space`.
+- When reusable content is used in pages within the parent space, all links remain relative to the space.
+- When reusable content is used across other spaces, we must resolve the refs relative to the containing space (not the parent).
+
+See the [ReusableContent](../components/DocumentView/ReusableContent.tsx) component for how we construct the `GitBookSpaceContext`, and [resolveContentRef](./references.tsx) for the `resolveContentRef` implementation.
+
+
+