I learnt it from the document marco, but I do not understand how to configure it as relative path, like fist level or second level as root node.
root The root node id. This is useful if you want to display only the descendants of a given node (which is the specified root). Depending on the value of the showRoot parameter, the tree displays the children of the root node on the first level or the root node itself. The entire tree is displayed if the root node is not specified. The format of a node identifier is entityType:entityReference, where the entity type can be for instance wiki, space, document. E.g.: wiki:xwiki, space:xwiki:Main, document:xwiki:Main.WebHome. Note that if you use an Entity Type other than document you must also set the showXXX parameter to true (where XXX is the Entity Type), e.g. showSpaces=true