Unfortunately, where field codes are not available in messages of certain types, it's because this is not possible, because of the nature of the fields.
Let's consider your specific example - a booking template does not allow the use of rental agreement related fields. While for most clients, most bookings only do have one (or no) rental agreement, that is not a limit of the system. It is possible to have multiple rental agreements on the same booking, for a variety of reasons - separate pet agreements, a requirement that every single guest personally sign the agreement not just whoever made the booking, etc.
Therefore, it's impossible to use those field codes in booking templates, because the system has no way of knowing which of the potentially many applicable agreements should be used to populate that field.
Similarly, consider the field codes related to payments. It's normal practice for one booking to have many payments. If you are generating a Payment type message, you know which specific payment it is for and can use those field codes. But if it's a Booking type message, how would the system know which of the many payments, with all sorts of different amounts, statuses, etc., should be used?
Hope this helps explain the limitations!
