With help from the users' mailing list:
Use a Tiki category 'tagging' (or other name) and select "Multiple-selection check-boxes". Add a Tiki child category for each topic you want to tag. Use the ID of category 'tagging' as "Parent Category".
The output is quite that what I wanted.
Filtering "Multiselect" field type leads to an error:
tracker field type not processed yet M