In a content repository, every node has one and only one primary node type. This node type defines, as mentioned, a set of restrictions on the child items of the node.
In addition to its single primary node type, a node may also have any number of mixin node types assigned to it. A mixin type is similar to a primary type in that its definition has the same parameters. It differs, though, in that it provides additional features to a node, beyond those defined in the node type proper.
Furthermore, while a primary node type can be “instantiated” as a node (i.e., that node’s structure is fully defined by its primary node type) this is not the case with mixin types. A mixin type cannot serve, by itself, to define the structure of a node; it just adds properties and child node requirements to a node that already has a primary node type.
A particular supported node type is either a primary type or a mixin type; it cannot be both.