What is the difference between custom property attributes and custom message tags?

Custom property attributes are most helpful for ensuring you only have 1 or 2 message rules for all properties. You can create an attribute called 'parking instructions' then insert the property specific information on each property. Then you can insert the message tag {{{parking_instructions}}} in your automated messages, saved replies, and rental agreements and it will pull the relevent property information when sending a message.

Custom message tags use the same content for all properties, but you can choose the booking sites you want that message to send to. So if you want to collect a security deposit for Booking.com and Uplisting and Google you can add the wording as a custom message tag, then insert that into your message rule and it will only send to bookings from those bookings sites (ie. it won't appear as part of the message rule sent to Airbnb guests).