I wouldn’t call it a “filter” - the XML hierarchy specifies a particular data instance, i.e. all keys in all lists must be included. Which elements that are keys (and how many there are of them in each list) depends on the data model, and RFC 6241 is a bit vague about that, since NETCONF was originally defined as independent of any particular data modeling language (it predates YANG). E.g it doesn’t talk about “lists” or “keys”, but the intro to the example does say:
Example: The <edit-config> examples in this section utilize a simple
data model, in which multiple instances of the <interface> element
can be present, and an instance is distinguished by the <name>
element within each <interface> element.