what is a junk attribute

From comments of struct JunkFilter:

A junk attribute is an attribute in a tuple that is needed only for
storing intermediate information in the executor, and does not belong
in emitted tuples. For example, when we do an UPDATE query,
the planner adds a "junk" entry to the targetlist so that the tuples
returned to ExecutePlan() contain an extra attribute: the ctid of
the tuple to be updated. This is needed to do the update, but we
don't want the ctid to be part of the stored new tuple!