Instructors sometimes train in a student's own aircraft. The system supports this by using a special aircraft with the registration "Other" and the type to "Various".
These aircraft are treated as a special case. When booking out, the actual registration and type are collected and confirmed when entering flight details. It is then these details that are transferred to the personal logbooks and training record.
Because the aircraft are not owned and managed by the school, it will not monitor maintenance or flag up warnings of missing or overlapping engine hours. The owner is responsible for keeping their own aircraft logs.
If you need more than one of these, use "Other 1", "Other 2" and so on. Most - if not all - schools will only need one at the most because the chances of booking a customer's own aircraft at the same time are so small.