Well, in my humble opinion, it is not a "legal" connection.
In my country, and under the suspect that this is a normal "home" aircon, (1 phase, split and low power one), normative commands that you MUST have an exclusive line from the main or the secondary board to the aircon. And it connects to the inner unit. There is a 3, 4 or 5 cable package that connects the inner with the outter unit, to provide power and command. That´s all. Intermediate boxes doesn´t exist.
Ok, that´s the normative, and good aircon installers follow it rigorously.
The rest of the universe? does what they want.