In the case of custom patterns it defines if the Node matched by this pattern are “kept” after the matching. The custom patterns simply forward the context of their caller query to their own pattern. For instance:

void rule()[
 Node -p-> Node ==>; // p is not a context
 Node -p-> Node ::>; // p is a context 
 Node (* -p-> Node *) ==>; // p is a context
]

A custom pattern p that is not a context makes all Nodes bound by its matching transient. Even the Node implicitly bound.