---
title: Reference
description: Lookup-oriented documentation for rtb-config covering the public API, the error type, and the Cargo features.
---
# Reference
Lookup-oriented pages: signatures, defaults, failure conditions. Read these when you
know what you are looking for and want the exact behaviour.
- **[API reference](api.md)**: every public type and method, its signature, its trait
bounds, and what it does when the input is wrong.
- **[Error reference](errors.md)**: every `ConfigError` variant, its `miette`
diagnostic code, and the exact conditions that produce it.
- **[Cargo features and dependencies](cargo-features.md)**: `hot-reload` and
`mutable`, what each adds, and the crate's MSRV.
Generated rustdoc for the same surface is on
[docs.rs/rtb-config](https://docs.rs/rtb-config). The pages here add the behaviour
that a signature does not show: merge order, which failures are silent, and what
each error actually means.
## Where to look for a given question
| What does this method return, and when does it fail? | [API reference](api.md) |
| Which layer wins when two of them set the same key? | [API reference](api.md#in-what-order-are-sources-merged) |
| What does this error message mean? | [Error reference](errors.md) |
| Which feature do I need for `watch_files` / `write`? | [Cargo features](cargo-features.md) |
| Why is my environment variable being ignored? | [Limitations](../explanation/limitations.md) |