---
title: "Project file"
description: "The container and the registry of definition sources"
url: "https://docs.justcat.it/reference/project-file/"
---
# Project file


The `.cat.yaml` file is what every CAT tool opens. This section documents the file itself — the container definitions can be written in, and the registry of the external lists CAT reads the rest of them from. The definitions themselves are documented under Data sources, Queries and Tests.