There are always 4 arguments on stack when
recv_internal is called.
recv_internal with less than 4 arguments you force FunC to ignore most deep variables (they still will be there, but your code will be unaware for the whole TVM execution).
in_msg_full - cell with raw message cell that contains all the flags and additional fields
in_msg_body - slice that only contain body of the message
Please check https://ton.org/docs/learn/tvm-instructions/tvm-overview#initialization-of-tvm for more informations.