Using the AppTree will build the XApplications variables for you. Selecting google chrome in the AppTree will set the variable XApplicaitons01 = 'Google Chrome' which will be picked up by the install applications task sequence step as you have set the base variable to XApplications. As you add more software into the AppTree, each additional selected application will be assigned XApplicatons02, XApplications03, etc. and also be picked up without further changes. There is no need to set XApplicatons variable on a collection.