Setting this property means that whenever a message is rendered (displayed) by Manila, it will override the normal Manila template to be the template whose name is the value of this variable in that message. In other words the template used depends on the message. Only one variable can set the template at any one time.
Manila can also set the template depending on the path in the site structure; and other Manila third party plugins can also set the template. It is undefined which will prevail when multiple attempts are made to set a template, so its not advisable to do so.