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

# Step 1: Understand User-Defined Functions

## Overview

User-Defined Functions let you define custom logic once as a named expression and reuse it wherever expressions are supported in Sift, including Calculated Channels, Rules, and other User-Defined Functions. They behave like built-in functions, can use built-in functions within their definitions, support optional inputs and constant values, and are version-controlled so logic can evolve safely.