The value determines how the node behaves at the client. Valid values and descriptions for the TreeViewPlusNodeClickBehavior enumeration:

• Click (default): no specific client behavior, but clicking on the node triggers the NodeClicked event.
• ExpandCollapse: clicking on the node's text causes the node to expand/collapse (normally users must click the expand/collapse button). Triggers the NodeCollapsed event or the NodeExpanded event.
• Link: the Text for the node displays as a hyperlink (underlined blue). The actual behavior is the same as Click, i.e., triggers a NodeClicked event, so any action must be handled via a server-side method attached to this event. (Note: currently this functionality is broken.)
• Selectable: hovering over the node changes its background color, and clicking on the node highlights it and triggers the NodeSelected event.
• None: nodes have no behavior or events, either on the client or on the server.

Reference