So, I don't know if this part was the same before, but the new regulations,
https://www.airbnb.com/terms/payments_terms ,
include this incredibly transparent and helpful statement:
--
2. Key Definitions
“Adjusted Exchange Rate” means the Base Exchange Rate plus a 3% fee charged by Airbnb for certain cross-border transactions.
--
The term is liberally used in the rest of the text.
No clarification about which are the "certain" transactions.
(Of course, needless to repeat, it shouldn't even be there. Online payments in other currencies are automatically taken care off in a way that ensures the final payment is the exact amount stated in the destination currency. I even bore myself by having to repeat the obvious.)