> This page location: Docs > Reference > Models
> Full index for agents: https://www.semantius.com/llms.txt

# Models

> Semantic data models in Semantius: entities, fields, relationships, and RBAC.

- **URL**: https://www.semantius.com/docs/models

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A **semantic model** is the heart of every Semantius platform. It describes your domain as data: the entities you care about, the attributes that define them, the relationships between them, and the rules that govern who can access what. Models are built and evolved through [agent skills](https://www.semantius.com/docs/agent-skills.md) and consumed by AI clients through [MCP connectors](https://www.semantius.com/docs/mcp-connectors.md).

## What a Model Contains

- **Entities**: the concepts in your domain (customers, orders, projects, devices).
- **Fields**: the attributes of each entity, with human-readable labels, descriptions, and UI hints.
- **Relationships**: how entities reference or own each other. Use `reference` for cross-entity links and `parent` for ownership and composition.
- **RBAC**: roles, permissions, and the access policies that govern reads and writes.

The model is **data, not code**. It can be inspected, edited, versioned, and manipulated by agents.

## Under the Hood

A semantic model is a single self-contained **markdown file**. Front-matter at the top carries identity (`system_slug`, `system_name`, `domain`) and discovery tags. The body lays out an overview, an entity summary table, a Mermaid entity-relationship diagram, per-entity field tables, a relationship summary, and an enumerations section.

Because it is just markdown, a model is:

- **Versioned** in the same Git repository as the rest of your code.
- **Reviewed** in pull requests, with diffs your team can read and reason about.
- **Discussed** by humans and agents in the same vocabulary, since both audiences read the same file.
- **Portable** between environments: the same file deploys to dev, staging, and production.

For a concrete example, see the [SaaS Expense Tracker model](https://www.semantius.com/models/saas_expense_tracker/model.md). It is fairly detailed (a real model usually has dozens of fields and a non-trivial relationship graph), but in day-to-day work you rarely open the file by hand: the [Create](https://www.semantius.com/docs/models/create.md), [Customize](https://www.semantius.com/docs/models/create.md#customize-or-extend-an-existing-model), [Deploy](https://www.semantius.com/docs/models/deploy.md), and [Optimize](https://www.semantius.com/docs/models/optimize.md) skills do the reading, writing, and diffing for you.

## The Model Lifecycle

Semantius treats a model as a living artifact, not a one-shot ship. Four agent skills span its full lifecycle, and each gets its own page:

- [Create](https://www.semantius.com/docs/models/create.md) a custom model from scratch by interviewing your domain.
- [Templates](https://www.semantius.com/docs/models/templates.md) are curated, ready-to-use models you can deploy as-is or customize first.
- [Deploy](https://www.semantius.com/docs/models/deploy.md) a model to a live Semantius instance, with safe diffing and conflict resolution.
- [Optimize](https://www.semantius.com/docs/models/optimize.md) by reverse-engineering a live module back into a markdown spec you can audit and improve.

Together they form a closed loop: design, deploy, customize in the platform, pull back down, refine, redeploy.

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## Related

- [Overview](https://www.semantius.com/docs/overview.md)
- [Structure](https://www.semantius.com/docs/models/structure.md)
- [Docs home](https://www.semantius.com/docs.md)
- [Full index for agents](https://www.semantius.com/llms.txt)