If you select subjects from the PUI top line to browse, you can (like other browse options) use header links in the right hand block to filter the records displayed.
I tried a number of examples on our instance of the PUI and found that filtering by repository isn't working. For example from this page https://archivesearch.lib.cam.ac.uk/subjects?q=%2A&op=OR&field=title&from_year=&to_year=&limit=subject&sort=title_sort%20asc&filter_fields=used_within_published_repository&filter_values=%2Frepositories%2F3
if I select the term 'atmosphere' I see results from other repositories to the one I have filtered by.
Was unable to locate an existing issue for this bug- but many apologies if I have missed something.