Logical notations in TT

Along with whatever display attributes are included in TT, will there be a
pre-defined (or extensible) set of ancillary information that can be
included with items of text.  Item #16 talks about speaker identification
via the IDREF attribute.  Would it be appropriate to include other
information in TT as well (for example, scene changes could be useful for
organizing blocks of subtitling/captioning) -- or would this information be
better stored in a separate file linked via the timecodes?