When the system applies a variable to set up the environment for a server, project, or step, it can do more than simply assign a value to it. You can choose an Action for each environment variable. The following actions are available:
Set: The default option. Assign As causes the system to assign the value you specify to the variable, creating the variable if it does not already exist.
Set if not set: This action assigns the value to the variable only if the variable does not already exist on the server when the system tries to set it. See How the System Applies Environments to Steps for more information.
Append: The system appends the value specified to any existing value for the variable.
Prepend: The system prepends the value to the current value.
Clear: The system clears the variable, setting it to null. The contents of the Value field are not used.
Delete / Unset: The system deletes the variable on the server. The contents of the Value field are not used.