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

# Core concepts overview

> The mechanics that apply to every Symmetry I-9 API call: authentication, webhooks and postback, pagination, and timezones. Read these once before building any workflow.

These concepts are true of the Symmetry I-9 API no matter which workflow you build. Understanding them once will make every other guide—setup, E-Verify, remote I-9, document verification, and compliance—easier to follow.

## Authentication

Symmetry I-9 uses two kinds of credentials: a system-level **API key** for server-to-server calls, and one-time **access URIs** that authenticate a user into an embedded session. Knowing which to use where is the foundation of every request.

<Card title="Authentication" icon="key" href="/i9/core-concepts/authentication">
  API keys, embedded access URIs, the API domain, and versioning.
</Card>

## Resources and their state

Every endpoint is scoped to a place in a fixed hierarchy, and an I-9's progress is tracked as three independent statuses rather than one.

<CardGroup cols={2}>
  <Card title="Resource Model & Identifiers" icon="sitemap" href="/i9/core-concepts/resource-model">
    How account, employer, employee, and submission nest—and which IDs are UUIDs.
  </Card>

  <Card title="Statuses & Lifecycle" icon="list-check" href="/i9/core-concepts/statuses-and-lifecycle">
    Section 1, countersign, and E-Verify status, and how to track each.
  </Card>
</CardGroup>

## Events: webhooks vs. postback

Symmetry sends you data in two complementary ways:

* **Webhooks** notify your platform in real time when something happens (an employee finishes Section 1, an E-Verify case updates, a document is expiring) so you can act or alert users.
* **Postback** delivers the completed Form I-9 payload to your employer's configured URL once the form is done.

<Card title="Webhooks" icon="bell" href="/i9/core-concepts/webhooks">
  Available events, sample payloads, and delivery retries.
</Card>

## Working with API data

<CardGroup cols={2}>
  <Card title="Pagination" icon="list-ol" href="/i9/core-concepts/pagination">
    Page through list endpoints with the `?page` query parameter.
  </Card>

  <Card title="Timezones" icon="clock" href="/i9/core-concepts/timezones">
    The ten supported timezones, and how to read I-9 dates and timestamps.
  </Card>
</CardGroup>

## Where to go next

Capabilities build on these concepts and are documented in their own sections:

<CardGroup cols={2}>
  <Card title="Embedded Experiences" icon="window" href="/i9/embedded-experiences/iframe-embed">
    Deliver the employee and administrator interfaces in an iframe.
  </Card>

  <Card title="Document Verification" icon="id-card" href="/i9/document-verification/overview">
    OCR-based screening of identity and work-authorization documents.
  </Card>
</CardGroup>