If you ever need to list all the variables in a ConfigMgr task sequence you can run the following VBScript.
Dim oTSEnv
Dim oVar
Set oTSEnv = CreateObject("Microsoft.SMS.TSEnvironment")
For Each oVar In oTSEnv.GetVariables
WScript.Echo " "
WScript.Echo " "& oVar & vbTab & oTSEnv(oVar) & vbTab
Next
Happy Deployment, Johan