If you fly in someone else's aircraft - perhaps when you revalidate a pilot's license - you probably want to record this in the booking system. But you won't want to create an aircraft record for use just once, or clutter the system with aircraft details you don't use regularly.
To handle this case, create an aircraft with the registration "Other" and set the type to "Various". You can use this aircraft for bookings just like any other, plus as an instructor, you can book out the aircraft without first making a booking (see the Booked Out screen). You can specify the actual aircraft registration and any other information you need in the aircraft notes and/or the training log.
Normally an aircraft will present warnings in the Aircraft list, such as when a service is due or when an aircraft has not been flown for a long time. Adding an aircraft named "Other" will suppress these warnings which cleans up the table.