> For the complete documentation index, see [llms.txt](https://docs.internetobject.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.internetobject.org/schema-definition-language/advanced-schema-concepts.md).

# Advanced Schema Concepts

- [TypeDef](https://docs.internetobject.org/schema-definition-language/advanced-schema-concepts/typedef.md): TypeDef — the fixed option contract that every MemberDef of a type is validated against.
- [MemberDef](https://docs.internetobject.org/schema-definition-language/advanced-schema-concepts/memberdef.md): MemberDef — defining one member's type, constraints, and optional/nullable/default behavior.
- [Open & Dynamic Schemas](https://docs.internetobject.org/schema-definition-language/advanced-schema-concepts/dynamic-schema.md): Strict and extensible schemas — accepting extra fields with the \* marker.
- [Union Types (anyOf)](https://docs.internetobject.org/schema-definition-language/advanced-schema-concepts/union-types.md): Members that accept more than one type, via anyOf.
- [Composition & Reuse](https://docs.internetobject.org/schema-definition-language/advanced-schema-concepts/composition.md): Composing and reusing schemas through references.


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