Halliday & Matthiessen (2014: 466):
Clauses with which (occasionally that), who or whose whose domain is a nominal group (the paratactic and cohesive agnates being personal references with he, she, it, they and their possessive equivalents); e.g.
||| This was the first English Department class at the University of Ibadan, || which had just been founded. |||
When the nominal group is non-final in the primary clause, the secondary clause is often enclosed, so as to follow immediately after it, as in
||| Yu, << who has been visiting Taiwan this week, >> did not elaborate. |||
Here the structure is α << =β >>; the angle brackets denote enclosure, doubled as always where the delimited element is a clause. (The paratactic agnate of an enclosed hypotactic elaborating clause would follow the primary clause, as in Yu did not elaborate; he has been visiting Taiwan this week.)