Halliday & Matthiessen (2014: 468):
There is one group of non-defining relative clauses which strictly speaking would belong with extension rather than elaboration; for example:||| She told it to the baker’s wife || who told it to the cook |||
Here the who stands for ‘and she’ and the clause is semantically an additive: the agnate paratactic variant would be … and she told it to the cook. Compare also (where the sense is ‘and in that case’):
||| It might be hungry || in which case it would be very likely to eat her up |||Note that such instances are not characterised by tone concord. Also extending rather than elaborating are possessives with whose or its variants (of whom/which), which do not further characterise the noun that constitutes their domain but add a new one related to it by possession; contrast elaborating come and meet Mary, whose birthday we’re celebrating (‘the girl whose…’) with extending the shop was taken over by an Indian, whose family came out to join him. But for most purposes these and all other non-defining relatives can be treated as elaborating clauses.