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

# CAST in 5 minutes

> The mental model that makes everything else legible.

Everything in CAST is an event or a projection of events. The flow below is the mental model the rest of the guide builds on.

```text The CAST flow theme={"dark"}
Event
  ↓
Policy Version
  ↓
Work Order
  ↓
Posting
  ↓
Case  (only on exception)
```

And running through all of it: **`lineage()`** — the immutable hash chain that binds every step back to the originating event.

<Note>
  Lineage is a property of the chain, not a sixth object. It is what makes a posting traceable to the event, decision, and policy version that produced it.
</Note>

## How to read the flow

<Steps>
  <Step title="An event is recorded">
    Something happens — an invoice arrives, a vendor confirms, a payment settles. Events are append-only. No updates, no deletes, ever.
  </Step>

  <Step title="A policy version governs it">
    Each event is pinned to the immutable policy version in force at the time. The policy decides what needs authorization, what auto-approves, and how exceptions are handled.
  </Step>

  <Step title="A work order holds the context">
    The work order is where the accounting work happens between an event and a ledger entry, tracking authorization, control, and settlement in parallel.
  </Step>

  <Step title="A posting is produced">
    Only after mutual authorization is complete does a journal-ready posting exist — carrying its full history back to the originating event.
  </Step>

  <Step title="A case opens on exception">
    A dispute, non-response, or budget breach becomes a governed case. Resolution events are appended; nothing prior is ever rewritten.
  </Step>
</Steps>

## The shift this represents

The ledger has historically been the system of record. In CAST, the **event log** is the system of record, and the ledger is a projection of it. That inversion is the heart of the architecture.

<Card title="The ledger is no longer the system of record" icon="arrow-right-arrow-left" href="/foundations/ledger-no-longer-system-of-record">
  Why the event log is primary and the ledger is derived.
</Card>