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

# Data customization

To help you keep an eye on errors and performance issues AppSignal allows you to provide the extra context by adding additional data to requests.

## Request Parameters

[Request parameters](/guides/custom-data/request-parameters) are both request query parameters and POST request payloads. AppSignal captures these parameters to help you understand what specific input may have triggered an error or performance issue.

## Request Session Data

[Request session data](/guides/custom-data/request-session-data) are values stored in a user's session, such as preferences and state. This data can help debug issues with specific states of a user session.

## Request Headers

[Request headers](/guides/custom-data/request-headers) include metadata about the HTTP request, such as the User-Agent, Content-Type, and custom headers. Capturing these can help you debug issues related to browsers, platforms, API versions, etc.

## Function parameters

[Function parameters](/guides/custom-data/function-parameters) are the parameters that background jobs or scripts start with. Capturing them can help you debug issues related to the initial context of those scripts.

## Custom Data

[Custom data](/guides/custom-data/custom-data) lets you attach any additional information you consider useful that does not fit in any of the other data categories.