When a developer wishes to CANCEL an existing booking, they should use the "Cancel Booking" message using the OTA standard endpoint "OTA_HotelResCancelPort" specification.  This allows the developer to also record the cancellation by offering these fields to record the event:

• Cancellation Number: (you can define your own number or let the system auto-generate one)
• Cancellation Fee: (you can set a fee, thought this has no bearing on any PMS product.
• Message: You can save a message about the cabncellation, such as: "guest cancelled by phone call".
• Phone: You can place a phoine number to call someone back or refer to the person recording teh cancellation.
• Cancelled By: Insert a valid PMS user name to record who did the cancellation.  If you have no idea whu to place here, use 'xml' so it's clear.

If a developer wished to MODIFY an existing booking, then they have two choices:

1. Either send the Rate and Availaility query to find out rate and avail for the new desired dates, and then cancel the existing booking make a New Booking.  This assures they get the rates desired for the modification, but doesn't guarantee accurate availability, based on the cancellation they are about to perform.  It could be a false positive.  
   
   For example, if there was no availabiity for a date range because the existing booking is still blocking the availability, the developer could receive a false positive that the modification wasn't possible due to no availability.
   
   
2. Or the developer could  and then send the Rate and Availaility query to find out rate and avail for the new desired dates, and make a New Booking.  This assures they get the accurate availability, based on the cancellation, but they won't know the actual rates for the modification until after they cancelled the booking. 
   
   Thus they could have cancelled a booking they wanted to save because it was priced better.  There is no perfect solution to this dilema, so choose your preferred option, but there is no solution for a direct 'modification' message.