> ## Documentation Index
> Fetch the complete documentation index at: https://docs.embedreach.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

> Tell Reach how to read your data as its core concepts

A **schema mapping** tells Reach how to interpret your [schema definitions](/api-reference/endpoint/get-partner-schema-definitions) in terms of its three canonical concepts — **contact**, **transaction**, and **location**. It's the step that says "the `email` field on this schema is the contact's email," "this schema is a `transactions_schema` and `transactionTotal` is the amount," and so on.

<Note>
  **Definition vs. mapping is the distinction partners most often trip on — read this before you map.**

  * A **definition** is *the shape of your data* (your field names, your structure).
  * A **mapping** is *how Reach reads that shape as its concepts*.

  Two steps, two parts of the UI. Start here:

  * [How Reach models your data](/data-sharing/data-model) — the concepts and the definition-vs-mapping split.
  * [Custom Schemas → Partner Schema Mappings](/data-sharing/custom-schemas#partner-schema-mappings) — the full mapping field reference for contacts, transactions, and locations.
</Note>

Define your schemas first, then map them. A mapping references a schema definition by ID, so the definition must exist before you can map it.