What is a deposit number (D-number)?

A deposit number is assigned by the Royal Library of Belgium. It is the code that a publisher is obliged to include on the front or on the back of a book to enable identification of the book and the publisher. 

Therefore, It must not be confused with ISBN, which has economic origins and is even not obligatory.

Is a deposit number obligatory for a PhD thesis? 

If you apply for an ISBN, then it is so to speak an official publication and you are also obliged to indicate a deposit number (and send two copies of the book to KBR for the legal deposit).

• If you apply for an ISBN through the Faculty Library of Bioscience Engineering, they will take care of the deposit number. (For your information: this is not typically offered by other faculty labraries.) 
• The deposit number (D-nummer) can be requested by a publisher. It follows the following pattern: the letter ‘D’ followed by the publication year, the number granted to the publisher and the serial number of the publication in the register of the publisher. For example: D/2018/14.289/4
• Authors who self-publish (that is who use the publisher only as a printer, but are consideredpublisher themselves), replace the publisher number and the serial number by the author's full name, followed by the word ‘uitgever’ (publisher). For example: D/2019/Jan Janssens, uitgever

If you do not apply for an ISBN, you indicate you publish only for internal use (jury, research group, family and friends) which indicates you don’t issue commercial editions, to  be offered to a broader public. In that case it is not obligatory to have a deposit number and to send the copies to KBR.