What is the reason for using EWR 25*25*1.6, instead of RHS 25*25*1.6 Thanks.

ERW is the name for thin walled RHS - same stuff just terminology

I think the thinnest wall RHS is 2mm.

As Mark says...basically it's all Electric resistance welded rolled hollow section...

ERWRHS

Usually the RHS is dropped in favour of square tube terminology for the thinner sections...and the ERW bit is dropped for the thicker stuff...but really other than the fact some is hot rolled and some is cold rolled it's pretty much made the same way.

There is more to it than that....but I think that's the basics.