The PHPCR odm stores meta information into the PHPCR node for each document. To avoid collisions and avoid issues with built-in types, every nodes gets the mixin type phpcr:managed that defines a phpcr:alias property. That way, meta information can be added to any node in a clean way.
See the README file of the project.
JCR defines node types which can be thought of a kind of schema for the repository's nodes. These types define the properties and children a node can have. JCR defines a set of node types that can be readily used. Some node types like nt:file restrict the set of allowed properties others like nt:unstructured allow any kind of properties and child nodes.
The PHPCR ODM stores with every node the alias of the document the node corresponds to. Therefore it adds a property named phpcr:alias to the node. If you use a node type like nt:file that does not allow free properties the node can not be saved when there are additional properties. Since the ODM uses the information stored in the property to construct the proper class when a node is read we need to find a way to store that property with all nodes managed by the ODM.
The node type phpcr:managed defines that property. When this node type is registered with the repository we can simply add that type to the node when the node is created.