What does a rule record about an absence?
Its dates, the work items it covers, any coverers or approver coverers, and an optional message.
The app has no absence-type or reason field, so it never records why someone is away. The optional Message Text you write is shown to colleagues, and to service desk customers on rules that share it, so do not put a reason there you would not want read.
Related pages
Privacy and Security - where this is covered in full.
Security and Privacy - more questions on this topic.