Expressions
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Expressions are a feature of 
Avatars .
They allow you to run various Actions on your avatar.
They can be used mainly through the Action Menu .
Expressions are player customized menus and actions meant to allow flexibility and easy access to avatar features.
You can find more information at 
Avatar Creators Docs
In Game Menu
The in game Menu expression is accessible through the Action Menu and is usable as well on desktop or in VR. It's made of all the different kind of buttons that the menu can have, and each type of menu will transfer an information to the avatar controllers to execute actions.
Those information are transferred through network to others users so information is synchronized.
Unity
The expressions are edited with the rest of the avatar in the Unity Editor.
Base Expressions
When no expressions are configured, a default expression menu with base animations is added to the avatar.
It contains the following entries :
- Wave
- Clap
- Point
- Cheer
- Dance
- Backflip
- Die
- Sadness
Custom Expressions
To add custom expressions, you need to create a Expression Menu file and a Expression Parameter file, then add them to the VRChat Avatar Descriptor.
The Menu will allow you to create the entries that will translate into in game menus for your avatar.
The parameters are the variables that are controlled by the Menu. You will also define technical information about those variables like if they need to be synchronized for other users.
