In order to help your invitee in the registration process, there are several texts on the registration page, that give an explanation to certain registration fields. These are standard texts, that can be managed and adjusted from the tool: Website Module, Pages, Register. The vocative case (in languages other than English) corresponds to the case you chose in Event details - Basic (Language).
Please note: If you switch from the formal to the informal case (or vice versa), the text will not change automatically.
Only new text will appear in the case of your choice.