Use

Many behaviors, such as the constructor/store, need to have a unique identifier for a list. This connection.listQuery method should return that.

Typically, a list's set identifier is a property on the list object. As example, a list of Todos might look like the following:

var dueTodos = todoConnection.getList({filter: {due: "today"}});
dueTodos; // [{_id: 5, name: "do dishes", due:"today"}, {_id: 6, name: "walk dog", due:"today"}, ...]
dueTodos[Symbol.for("can.listQuery")]; //-> {filter: {due: "today"}}
todoConnection.listQuery(dueTodos); //-> {filter: {due: "today"}}

In the above example the listQueryProp would be the default @can.listQuery.