Imagine a scattering process a+b->c+d->e+f+g+h. After decomposing the four-body final phase space into product of 2->2 scattering process. And then, the amplitude requires me to calculate the scalar-product of $k_{a}\cdot p_{e}$, but the four-momentum $k_{a}$ and $p_{e}$ should be defined in their own or parent's frame.So in order to get the right expression of scalar-product of them, should I boost one particle to the frame where the another is and the calculate the scalar-product $k_{a}^*\cdot p_{e}$ (the $k_{a}^*$ is the boosted four-momentum of particle a ?