# MediaTailor dynamic ad variables for ADS requests
<a name="variables"></a>

AWS Elemental MediaTailor uses dynamic ad variables to pass information from your viewing session to the ad decision server (ADS). This information helps the ADS select the most relevant ads for your viewers.

This section provides an overview of dynamic ad variables and links to specific implementation guides. For step-by-step configuration instructions, see the individual topics below.

**Dynamic variable types**  
MediaTailor supports four types of dynamic variables:
+ **Session variables** – Automatically generated values like session ID and SCTE-35 data. See [MediaTailor session variables for ADS requests](variables-session.md).
+ **Player variables** – Custom parameters sent by your video player. See [MediaTailor player variables for ADS requests](variables-player.md).
+ **Domain variables** with **Configuration aliases** – Dynamic URL domains for multi-origin configurations. 
+ **Configuration aliases** – Predefined mappings for dynamic variable replacement. See [Configuration aliases](configuration-aliases-overview.md).

**Common use cases**  
Use dynamic ad variables to:
+ Pass viewer demographics and preferences to your ADS
+ Route requests to different origins based on geographic location
+ Enable time-shifted viewing with MediaPackage integration
+ Implement A/B testing and failover scenarios

The following sections provide additional detail about using dynamic ad variables with MediaTailor.

**Topics**
+ [Session variables](variables-session.md)
+ [Player variables](variables-player.md)
+ [Domain variables](variables-domains.md)
+ [Configuration aliases](configuration-aliases-overview.md)
+ [Passing ADS parameters](passing-paramters-to-the-ads.md)
+ [Parameter routing](parameter-routing-behavior.md)
+ [MediaPackage integration](mediapackage-integration-param.md)
+ [Session behavior](parameter-session-behavior.md)
+ [Parameter reference](parameter-comprehensive-reference.md)
+ [Parameter troubleshooting](parameter-troubleshooting.md)
+ [Alias troubleshooting](configuration-aliases-troubleshooting.md)

For parameter formatting requirements and troubleshooting, see [MediaTailor parameter reference and limitations](parameter-comprehensive-reference.md) and [MediaTailor parameter troubleshooting guide](parameter-troubleshooting.md).