The hierarchy of a namespace is composed of nodes. Each node can have
children (called sub-nodes). Nodes are, in fact, ObjectNode.
Just like in a filesystem, nodes have different types.
ObjectNode can be either SubjectNode or
ActionNode. ObjectNode is an abstract data type,
which describes the namespace structure.
SubjectNode is a type for storing most Slide information.
A subject can be a file, a directory or a principal.
ActionNode is a type for defining available actions on Slide
objects. In Slide, actions can be dynamically placed in the namespace
to provide flexible security policy definition.
The type of the node determines which roles the node has. The Security
page has more details on roles and role resolution.
