Hello Per Andersson,
Thank you for reaching out to Microsoft!!
For the documentation or references regarding which attributes can be updated through API-driven provisioning to on-premises Active Directory using bulk upload, please find the below links which might be helpful.
Link: API-driven provisioning
For bulk uploading, please refer this section: Bulk Upload
Hope this helps.
If the answer is helpful, please click Accept Answer and kindly upvote it. If you have any further questions about this answer, please click Comment.