In the manual, the item bundle section (https://koha-community.org/manual/latest/fr/html/circulation.html#circulating-bundles) states "The bundle functionality will only be displayed if you create a catalogue record with the Leader, (000 MARC tag) position 7 set to ‘c’ for Collection." and displays a picture from MARC21 framework This exact configuration setting is working for UNIMARC ("c" in pos.7 of leader), but this is not obvious as many settings in Koha are MARC-flavour-dependant