What is the difference between boundary and domain when assigning materials?

Hi, I'm new to comsol and the answer maybe obvious to some of you, but here's my situation. I want to design a piezoresistive sensor. In the model library, there is a similar sensor example where the piezoresistive material is applied to a boundary. But I don't understand why. To my understanding, it should be assign to a domain.

I will also appreciate if anybody has a link to a site describing the differences.


Thanks,
Mahatasin