Boundaries and material differences in HFSS

Hello
In a design antenna video I see we should define material of an antenna and select boundary to PEC. my questions is why should we define PEC for antenna as boundary? because the boundary of antenna isn't air and material is PEC?

you are defining the metallic material as PEC. the radiation boundary is air or vacuum. If you are asking why PEC and not copper this is because copper is lossy and simulating copper needs more time. So it is assumed that the metals are PEC.