The example demostrates a sample usage of the RadAjaxManager in Master pages. You can have a single RadAjaxManager class on the master page which will be responsible for "ajaxifying" and updating the controls inside the content page. In this example, the manager "ajaxifies" a treeview which resides in the master page and updates a grid in a content placeholder. In this case, the AjaxSettings (for the manager class) are applied only programatically since the designer cannot be used for master/content pages.

You can review the following help topics, which elaborate further on the matter:
AJAX and MasterPage
MasterPages: Ajaxify and update controls in master and content pages