Names of fields whose value actually differs.
Computed from a value comparison rather than from the dirty flag: a field can be assigned its existing value and still read as dirty, and a filter that fired on that would be reporting a change that did not happen.
True when the record is being created, so every field is "new" and none of them changed.
Field values as they are now.
Field values as they were when the record was loaded. Empty on a create — there is no "before".
The before/after picture of one save, as seen by a filter.