[U-Boot] [spl framework] how spl framework resolve one .config for 2 different compiles?

hi, I have a question that how spl framework resolve one .config for 2 different compiles. Maybe i am not fully understand SPL framework, for example, CONFIG_SPL_BUILD=y will be defined in both compiles, but i think it's only needed for SPL image build, no need for the other image.