If creating an Event via the nextcloud Ui, the Caldav server response looks like this (output shortened):
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
BEGIN:VTIMEZONE
TZID:Europe/Berlin
BEGIN:DAYLIGHT
TZOFFSETFROM:+0100
TZOFFSETTO:+0200
TZNAME:CEST
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=3
END:DAYLIGHT
BEGIN:STANDARD
SUMMARY:Test
.....
CATEGORIES:Jg. 5 <-- // here
CATEGORIES:Jg. 6 <-- // here
....
END:VEVENT
END:VCALENDAR
If the event is created in Thunderbird it looks like this (output shortened):
BEGIN:VCALENDAR
VERSION:2.0
CALSCALE:GREGORIAN
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
DTSTART:19700329T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
DTSTART:19701025T030000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CREATED:20200813T111910Z
LAST-MODIFIED:20200817T084432Z
DTSTAMP:20200817T084432Z
UID:584589d3-54a6-4f5a-bd4c-03e478938b07
SUMMARY:Test
CATEGORIES:Jg. 5,Jg. 6 <-- // here
....
END:VEVENT
END:VCALENDAR
As you can see, Nextcloud creates for each category it's own field, while Thunderbird separates each category with a comma (,).
Is this right?
As per rfc 5545 section 3.8.1.2, the CATEGORIES property can have multiple, comma-separated values. This is what Thunderbird does.
At the same time, there are a number of rfc 5545 properties that are allowed to be specified more than once, e.g. ATTENDEE. Therefore parsers have learned to deal with multiple properties. Looking at rfc 7986 section 5.6, multiple properties for CATEGORIES are explicitly allowed.
It is possible that some parsers may internally turn multiple CATEGORIES properties into a single one (or vice versa?), which would be noticeable when editing the event.