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The X-ABLabel field can be used to create custom labels in an contact. This is useful when none of the default labels really fit. For example, a contact for the emergency services can have labels such as Fire, Police, etc.

Such a contact is shown here, created in the Nextcloud Contacts web interface:

Resulting vCard:

BEGIN:VCARD
VERSION:3.0
PRODID:-//Sabre//Sabre VObject 4.1.6//EN
UID:35dd880f-972e-4e56-846c-991839c43e96
REV;VALUE=DATE-AND-OR-TIME:20200603T133120Z
FN:Emergency Services (Nextcloud)
ADR;TYPE=HOME:;;;;;;
EMAIL;TYPE=HOME:
NEXTCLOUD1.TEL;TYPE="HOME,VOICE":110
NEXTCLOUD2.TEL;TYPE="HOME,VOICE":112
NEXTCLOUD3.TEL;TYPE="HOME,VOICE":113
ORG:Emergency Services (Nextcloud)
NEXTCLOUD1.X-ABLABEL:Fire department
NEXTCLOUD2.X-ABLABEL:Police
NEXTCLOUD3.X-ABLABEL:Ambulance
END:VCARD

You can do the same thing in Google Contacts:

Corresponding vCard:

BEGIN:VCARD
VERSION:3.0
item1.TEL:110
item1.X-ABLabel:Fire dept
item2.TEL:112
item2.X-ABLabel:Police
item3.TEL:113
item3.X-ABLabel:Ambulance
item4.ORG:Emergencies (Google)
item4.X-ABLabel:
CATEGORIES:myContacts
END:VCARD

However, when synced to Evolution (using CardDAV protocol for Nextcloud and whatever a Google account in GOA uses), the informative labels are not shown, instead all the numbers are shown as generic Telephone entries:

Next, if I edit these contacts in Evolution, by adding an unrelated field such as a note, and sync it back to the server, the labels are lost.

In Nextcloud, they now show up as Select type:

In Google, they have become Other:

The labels are still there in the Nextcloud vCard, but they have been "disconnected" from the phone number entries, so they are for all practical purposes lost:

BEGIN:VCARD
VERSION:3.0
UID:35dd880f-972e-4e56-846c-991839c43e96
X-EVOLUTION-FILE-AS:Emergency Services (Nextcloud)
PRODID:-//Sabre//Sabre VObject 4.1.6//EN
REV;VALUE="DATE-AND-OR-TIME":20200603T133120Z
FN:Emergency Services (Nextcloud)
ORG:Emergency Services (Nextcloud);;
NEXTCLOUD1.X-ABLABEL:Fire department
NEXTCLOUD2.X-ABLABEL:Police
NEXTCLOUD3.X-ABLABEL:Ambulance
NOTE:This is a note
X-MOZILLA-HTML:FALSE
TEL:110
TEL:112
TEL:113
END:VCARD

In the Google vCard, on the other hand, they labels are simply gone. Perhaps their sync server implementation detected that they were orphaned, and removed them. (It seems to have pruned other stuff as well, e.g., X-EVOLUTION-FILE-AS).

BEGIN:VCARD
VERSION:3.0
FN:Emergencies (Google)
N:(Google);Emergencies;;;
TEL:110
TEL:112
TEL:113
item1.ORG:Emergencies (Google)
item1.X-ABLabel:Other
NOTE:Don't prank call!
CATEGORIES:myContacts
END:VCARD

I'm using evolution-3.36.2-1.fc32.x86_64.rpm on Fedora 32.