User wall heat flux coefficient specification
 

Hello

I am trying to understand the user wall heat flux specification, to set it up at a fluid / solid interface.
From the manual, it is written that to specify a user coefficient will not affect the amount of the heat flux, but how much the heat flux goes toward influencing the temperature of the fluid or solid.

What coefficient will have the strongest effect?
Does the value A,B,C,D has to be the same on the 'two side' of the interface, or implementing them only on one domain is enough?
How to modify the overall heat flux from the interface? (Or how to increase or decrease the total heat transfer coefficient?)

Thank you for your tips