> ## 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 3: Apply User-Defined Functions

## Overview

Once created, a User-Defined Function becomes available anywhere expressions are used in Sift, including Rules, Calculated Channels, and other User-Defined Functions. In the expression editor, User-Defined Functions appear in the **Expression Syntax** panel under the **User-Defined Functions** section.

## Using User-Defined Functions in expressions

When defining an expression, you can reference a User-Defined Function just like a built-in function. In the examples below, `$1` refers to **camera\_1.temperature**. For example, in a Calculated Channel:

```
fl_tutorial_7_user_defined_function($1)
```

Or in a Rule, as part of a logical condition:

```
fl_tutorial_7_user_defined_function($1) > 86
```

Because User-Defined Functions are defined once and reused across expressions, updating the function's logic applies the change centrally without requiring updates everywhere it is used.