By law, "Prince/sse" is the legal
title and "de Habsbourg-Lorraine" is the legal
surname of the branches descended from Prince and Princesse Carl Ludwig de Habsbourg-Lorraine (which indeed are from a different line of the family).
As shown in the rules of the national registration, the only titles of nobility that are registered with a designation are "King/Queen of the Belgians", "Prince/ss of Belgium" and "Duke/Duchess of Brabant". All other titles of nobility are legally in the form "Prince/ss", "Duke/Duchess" etc. without further designation, even if they are associated with a surname (which is a separate thing) such as "de Habsbourg-Lorraine".
https://www.ibz.rrn.fgov.be/fileadm...ructions/liste-TI/TI012_Titre_de_noblesse.pdf
code titre : le titre de noblesse est codé par 2 chiffres conformément au tableau ci-dessous :
01
Prince
02
Princesse
03
Duc
04
Duchesse
05
Marquis
06
Marquise
07
Comte
08
Comtesse
09
Vicomte
10
Vicomtesse
11
Baron
12
Baronne
13
Chevalier
15
Ecuyer
17
Archiduc
18
Archiduchesse
19
Grand-Duc
20
Grande-Duchesse
21
Roi des Belges
22
Reine des Belges
23
Prince de Belgique
24
Princesse de Belgique
25
Duc de Brabant,
Prince de Belgique
26
Duchesse de Brabant,
Princesse de Belgique
The question is whether Habsburg-Este is the surname of the branch issuing from Prince Lorenz.
Looking once again at Princess Anna Astrid's birth registration (
link), it lists her as
Son Altesse Impériale et Royale la Princesse Anna Astrid Marie Archiduchesse d'Autriche-Este (Habsbourg-Lorraine)
(Her Imperial and Royal Highness Princess Anna Astrid Marie Archduchess of Austria-Este (Habsburg-Lorraine))
As seen in the regulations above, "Princesse" and "Archiduchesse" are recognized as legal titles in the national registration, whereas "Archiduchesse d'Autriche-Este" is not. Under the legal rules, "d'Autriche-Este" is part of the surname: "d'Autriche-Este (Habsbourg-Lorraine)".