This depends on which kind of structural element is being calculated. In addition it also depends on the type of stress that the structural element is subjected to. We would need to know how the built-up box is going to be used.
If it is a column, as it appears to be from the type of detail of the construction section, the entire surface of the steel section would be used for the compression calculus.
I hope it gives you some guidance and if not, I hope someone else will solve the doubt.
