In JSR170, a node can only be the child node of one other node. This makes it impossible to file nodes under multiple taxonomies, i.e. defining multiple paths to get to the same node. JSR283 hence introduces the (optional) concept of shareable nodes. A shareable node is a node which can share its properties and children with other nodes. A node type is made shareable by adding the mixin mix:shareable.
It is important to note that by adding shareable nodes, the path of a node does not identify a unique node anymore. Rather, there may be different paths to reach a node, such as A/S1/Y and B/S2/Y both identify node Y in the above example. It is up to the particular implementation to define a deemed path, i.e. the default return value of Node.getPath().