# Rules [#rules]

Rules derive facts from other facts and parameter services. They are the
source-backed calculation steps behind calculator reports.

## When this matters [#when-this-matters]

You usually do not call rules directly as an API or SDK consumer. You need the
model when you inspect graph metadata, review diagnostics or contribute tax
behaviour.

## How it works [#how-it-works]

```txt
GrossPay
  -> taxable pay rule
  -> PAYG withholding rule
  -> net pay rule
  -> TakeHomePayReport
```

Rules have descriptors so TaxKit can expose graph edges, source references
and required facts through metadata endpoints.

## Parameters [#parameters]

Tax-year rates and thresholds are services. The same algorithm can be paired
with different parameter layers for different effective periods.

## Related concepts [#related-concepts]

* [Calculators](/concepts/calculators)
* [Diagnostics](/concepts/diagnostics)
* [Rules and parameters architecture](https://github.com/crcorbett/taxkit/blob/a151e51e8a30247526fa93412df046955846eca4/docs/architecture/rules-and-parameters.md)